index.html

<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>RDF Surfaces Primer</title>
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in § N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *     -> ins/del.c### for candidate and proposed changes (amendments)
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *   - .correction for "candidate corrections"       (div, aside, details, section)
 *   - .addition   for "candidate additions"         (div, aside, details, section)
 *   - .correction.proposed for "proposed corrections" (div, aside, details, section)
 *   - .addition.proposed   for "proposed additions"   (div, aside, details, section)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Outdated warning for old specs
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/* color variables included separately for reliability */

/******************************************************************************/
/*                                    Body                                    */
/******************************************************************************/

	html {
	}

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;			  /* limit line length to 50em for readability   */
		margin: 0 auto;				/* center text within page                    */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag    */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		color: black;
		color: var(--text);
		background: white top left fixed no-repeat;
		background: var(--bg) top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em; }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border: solid var(--logo-bg);
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		background: var(--logo-bg);
		color: white;
		color: var(--logo-text);
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		background: var(--logo-active-bg);
		border-color: #c00;
		border-color: var(--logo-active-bg);
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small; }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 3;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;

			color: #707070;
			color: var(--tocnav-normal-text);
			background: white;
			background: var(--tocnav-normal-bg);
		}
		#toc-nav > a:hover,
		#toc-nav > a:focus {
			color: black;
			color: var(--tocnav-hover-text);
			background: #f8f8f8;
			background: var(--tocnav-hover-bg);
		}
		#toc-nav > a:active {
			color: #c00;
			color: var(--tocnav-active-text);
			background: white;
			background: var(--tocnav-active-bg);
		}

		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			color: black;
			color: var(--tocsidebar-text);
			background: inherit;
			background-color: #f7f8f9;
			background-color: var(--tocsidebar-bg);
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
			box-shadow: -.1em 0 .25em var(--tocsidebar-shadow) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
			color: var(--tocsidebar-heading-text);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;	/* Inherit the font family. */
		line-height: 1.2;		/* Keep wrapped headings compact */
		hyphens: manual;		/* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		color: var(--heading-text);
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#profile-and-date {
		/* #profile-and-date is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > :not(.head) + hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		color: black;
		color: var(--hr-text);
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	border-left: 0.5em solid #DEF;
	border-left: 0.5em solid var(--algo-border);
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	 padding: .5em;
	 border: thin solid #ddd;
	 border: thin solid var(--algo-border);
	 border-radius: .5em;
	 margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	 margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	 margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	margin-left: -2em;
	}
	dl.switch {
	padding-left: 2em;
	}
	dl.switch > dt {
	text-indent: -1.5em;
	margin-top: 1em;
	}
	dl.switch > dt + dt {
	margin-top: 0;
	}
	dl.switch > dt::before {
	content: '\21AA';
	padding: 0 0.5em 0 0;
	display: inline-block;
	width: 1em;
	text-align: right;
	line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: inherit;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del {
		color: #aa0000;
		color: var(--del-text);
		background: transparent;
		background: var(--del-bg);
		text-decoration: line-through;
	}
	ins {
		color: #006100;
		color: var(--ins-text);
		background: transparent;
		background: var(--ins-bg);
		text-decoration: underline;
	}

	/* for amendments (candidate/proposed changes) */

	.amendment ins, .correction ins, .addition ins,
	ins[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment del, .correction del, .addition del,
	del[class^=c] {
		text-decoration-style: dotted;
	}
	.amendment.proposed ins, .correction.proposed ins, .addition.proposed ins,
	ins[class^=c].proposed {
		text-decoration-style: double;
	}
	.amendment.proposed del, .correction.proposed del, .addition.proposed del,
	del[class^=c].proposed {
		text-decoration-style: double;
	}

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		hyphens: none;
		text-transform: none;
		text-align: left;
		text-align: start;
		font-variant: normal;
		orphans: 3;
		widows: 3;
		page-break-before: avoid;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

	/* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		color: var(--a-normal-text);
		text-decoration: underline #707070;
		text-decoration: underline var(--a-normal-underline);
		text-decoration-skip-ink: none;
	}
	a:visited {
		color: #034575;
		color: var(--a-visited-text);
		text-decoration-color: #bbb;
		text-decoration-color: var(--a-visited-underline);
	}

	/* Indicate interaction with the link */
	a[href]:focus,
	a[href]:hover {
		text-decoration-thickness: 2px;
	}
	a[href]:active {
		color: #c00;
		color: var(--a-active-text);
		text-decoration-color: #c00;
		text-decoration-color: var(--a-active-underline);
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	img, svg {
		/* Intentionally not color-scheme aware. */
		background: white;
	}

	/* For autogen numbers, add
	  .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,	.sidefigure img,	figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
		height: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em;
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em; }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote,
	.amendment, .correction, .addition {
		margin: 1em auto;
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	blockquote > :first-child,
	.note  > p:first-child,
	.issue > p:first-child,
	.amendment > p:first-child,
	.correction > p:first-child,
	.addition > p:first-child {
		margin-top: 0;
	}
	blockquote > :last-child,
	.note  > p:last-child,
	.issue > p:last-child,
	.amendment > p:last-child,
	.correction > p:last-child,
	.addition > p:last-child {
		margin-bottom: 0;
	}


	.issue::before, .issue > .marker,
	.example::before, .example > .marker,
	.note::before, .note > .marker,
	details.note > summary > .marker,
	.amendment::before, .amendment > .marker,
	details.amendment > summary > .marker,
	.addition::before, .addition > .marker,
	addition.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	correction.amendment > summary > .marker
	{
		text-transform: uppercase;
		padding-right: 1em;
	}

	.example::before, .example > .marker {
		display: block;
		padding-right: 0em;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
		border-color: var(--blockquote-border);
		background: transparent;
		background: var(--blockquote-bg);
		color: currentcolor;
		color: var(--blockquote-text);
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #e05252;
		border-color: var(--issue-border);
		background: #fbe9e9;
		background: var(--issue-bg);
		color: black;
		color: var(--issue-text);
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		color: #831616;
		color: var(--issueheading-text);
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	  or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #e0cb52;
		border-color: var(--example-border);
		background: #fcfaee;
		background: var(--example-bg);
		color: black;
		color: var(--example-text);
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		color: #574b0f;
		color: var(--exampleheading-text);
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	  or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52e052;
		border-color: var(--note-border);
		background: #e9fbe9;
		background: var(--note-bg);
		color: black;
		color: var(--note-text);
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary {
		color: hsl(120, 70%, 30%);
		color: var(--noteheading-text);
	}
	/* Add .note::before { content: "Note "; } for autogen label,
	  or use class="marker" to mark up the label in source. */

	details.note[open] > summary {
		border-bottom: 1px silver solid;
		border-bottom: 1px var(--notesummary-underline) solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		border-color: var(--assertion-border);
		background: #EEE;
		background: var(--assertion-bg);
		color: black;
		color: var(--assertion-text);
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-color: var(--advisement-border);
		border-style: none solid;
		background: #fec;
		background: var(--advisement-bg);
		color: black;
		color: var(--advisement-text);
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement::before, .advisement > .marker {
		color: #b35f00;
		color: var(--advisementheading-text);
	}

/** Amendment Box *************************************************************/

	.amendment, .correction, .addition {
		border-color: #330099;
		border-color: var(--amendment-border);
		background: #F5F0FF;
		background: var(--amendment-bg);
		color: black;
		color: var(--amendment-text);
	}
	.amendment.proposed, .correction.proposed, .addition.proposed {
		border-style: solid;
		border-block-width: 0.25em;
	}
	.amendment::before, .amendment > .marker,
	details.amendment > summary::before, details.amendment > summary > .marker,
	.correction::before, .correction > .marker,
	details.correction > summary::before, details.correction > summary > .marker,
	.addition::before, .addition > .marker,
	details.addition > summary::before, details.addition > summary > .marker {
		color: #220066;
		color: var(--amendmentheading-text);
	}
	.amendment.proposed::before, .amendment.proposed > .marker,
	details.amendment.proposed > summary::before, details.amendment.proposed > summary > .marker,
	.correction.proposed::before, .correction.proposed > .marker,
	details.correction.proposed > summary::before, details.correction.proposed > summary > .marker,
	.addition.proposed::before, .addition.proposed > .marker,
	details.addition.proposed > summary::before, details.addition.proposed > summary > .marker {
		font-weight: bold;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: hsla(40,100%,50%,0.95);
		background: var(--warning-bg);
		color: black;
		color: var(--warning-text);
		padding: .75em 1em;
		border: red;
		border: var(--warning-border);
		border-style: solid none;
		box-shadow: 0 2px 8px black;
		text-align: center;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 0;
		right: 0;
		bottom: 2em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #def;
		background: var(--def-bg);
		margin: 1.2em 0;
		border-left: 0.5em solid #8ccbf2;
		border-left: 0.5em solid var(--def-border);
		color: black;
		color: var(--def-text);
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
		border-bottom: 1px solid var(--defrow-border);
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def td.footnote {
		padding-top: 0.6em;
	}
	table.def td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		<table class="data"> highlights structural relationships in a table
		when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		Use class="complex data" for particularly complicated tables --
		(This will draw more lines: busier, but clearer.)

		Use class="long" on table cells with paragraph-like contents
		(This will adjust text alignment accordingly.)
		Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-color: var(--datacell-border);
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		border-top: 1px solid var(--datacell-border);
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		border: 1px solid var(--datacell-border);
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
		vertical-align: baseline;
		text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding: 0.1rem 1px 0;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Switch to using border-bottom for underlines */
		text-decoration: none;
		border-bottom: 1px solid;
		/* Reverse color scheme */
		color: black;
		color: var(--toclink-text);
		border-color: #3980b5;
		border-color: var(--toclink-underline);
	}
	.toc a:visited {
		color: black;
		color: var(--toclink-visited-text);
		border-color: #054572;
		border-color: var(--toclink-visited-underline);
	}
	.toc a:focus,
	.toc a:hover {
		background: rgba(75%, 75%, 75%, .25);
		background: var(--a-hover-bg);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
	}
	.toc {
		line-height: 1.1em;
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li			{ font-weight: bold;   }
	.toc > li li		 { font-weight: normal; }
	.toc > li li li	  { font-size:   95%;	}
	.toc > li li li li	{ font-size:   90%;	}
	.toc > li li li li li { font-size:   85%;	}

	/* @supports not (display:grid) { */
		.toc > li			{ margin: 1.5rem 0;	}
		.toc > li li		 { margin: 0.3rem 0;	}
		.toc > li li li	  { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}
		.toc > li li li li .secno { font-size: 85%; }
		.toc > li li li li li .secno { font-size: 100%; }

		.toc li {
			clear: both;
		}

		:not(li) > .toc			 { margin-left:  5rem; }
		.toc .secno				 { margin-left: -5rem; }
		.toc > li li li .secno	  { margin-left: -7rem; }
		.toc > li li li li .secno	{ margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc			 { margin-left:  4rem; }
			.toc .secno				 { margin-left: -4rem; }
			.toc > li li li			 { margin-left:  1rem; }
			.toc > li li li .secno	  { margin-left: -5rem; }
			.toc > li li li li .secno	{ margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
		/* Loosen it on wide screens */
		@media screen and (min-width: 78em) {
			body:not(.toc-inline) :not(li) > .toc			 { margin-left:  4rem; }
			body:not(.toc-inline) .toc .secno				 { margin-left: -4rem; }
			body:not(.toc-inline) .toc > li li li			 { margin-left:  1rem; }
			body:not(.toc-inline) .toc > li li li .secno	  { margin-left: -5rem; }
			body:not(.toc-inline) .toc > li li li li .secno	{ margin-left: -6rem; }
			body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			  comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
			border-bottom: 3px solid transparent;
			margin-bottom: -3px;
		}
		#toc .content:hover,
		#toc .content:focus {
			background: rgba(75%, 75%, 75%, .25);
			background: var(--a-hover-bg);
			border-bottom-color: #054572;
			border-bottom-color: var(--toclink-underline);
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index	  { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li	{ margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em; }
	ul.index dl	{ margin-top: 0; }
	ul.index dt	{ margin: .2em 0 .2em 20px;}
	ul.index dd	{ margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li a + span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
			color: var(--indexinfo-text);
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		color: black;
		color: var(--indextable-hover-text);
		background: #f7f8f9;
		background: var(--indextable-hover-bg);
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/** Outdated warning **********************************************************/

.outdated-spec {
	color: black;
	color: var(--outdatedspec-text);
	background-color: rgba(0,0,0,0.5);
	background-color: var(--outdatedspec-bg);
}

.outdated-warning {
	position: fixed;
	bottom: 50%;
	left: 0;
	right: 0;
	margin: 0 auto;
	width: 50%;
	background: maroon;
	background: var(--outdated-bg);
	color: white;
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	color: var(--outdated-text);
	border-radius: 1em;
	box-shadow: 0 0 1em red;
	box-shadow: 0 0 1em var(--outdated-shadow);
	padding: 2em;
	text-align: center;
	z-index: 2;
}

.outdated-warning a {
	color: currentcolor;
	background: transparent;
}

.edited-rec-warning {
	background: darkorange;
	background: var(--editedrec-bg);
	box-shadow: 0 0 1em;
}

.outdated-warning button {
	position: absolute;
	top: 0;
	right:0;
	margin: 0;
	border: 0;
	padding: 0.25em 0.5em;
	background: transparent;
	color: white;
	color: var(--outdated-text);
	font:1em sans-serif;
	text-align:center;
}

.outdated-warning span {
	display: block;
}

.outdated-collapsed {
	bottom: 0;
	border-radius: 0;
	width: 100%;
	padding: 0;
}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}

		.outdated-warning {
			position: absolute;
			border-style: solid;
			border-color: red;
		}

		.outdated-warning input {
			display: none;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}


/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good item positioning:
		  "content column" is 50ems wide at max; less on smaller screens.
		  Extra space (after ToC + content) is empty on the right.

		  1. When item < content column, centers item in column.
		  2. When content < item < available, left-aligns.
		  3. When item > available, fills available + scroll bar.
		*/
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			* http://lea.verou.me/2012/04/background-attachment-local/
			*
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
						top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
						top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
						white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
  <meta content="Bikeshed version d765c696b, updated Fri Mar 8 15:58:52 2024 -0800" name="generator">
  <meta content="dark light" name="color-scheme">
<style>
table {
  margin: 25px auto;
  border-collapse: collapse;
  border: 1px solid #eee;
  border-bottom: 2px solid #005A9C;
}
table tr:hover {
  background: #f4f4f4;
}
table tr:hover td {
  color: #555;
}
table th, table td {
  color: #999;
  border: 1px solid #eee;
  padding: 12px 12px;
  border-collapse: collapse;
}
table th {
  background: #005A9C;
  color: #fff;
}
table tbody > tr > th {
  background: #fff;
  color: #005A9C;
}
table th.last {
  border-right: none;
}
</style>
<style>/* Boilerplate: style-autolinks */
.css.css, .property.property, .descriptor.descriptor {
    color: var(--a-normal-text);
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}

@media (prefers-color-scheme: dark) {
    :root {
        --selflink-text: black;
        --selflink-bg: silver;
        --selflink-hover-text: white;
    }
}
</style>
<style>/* Boilerplate: style-colors */
/* Any --*-text not paired with a --*-bg is assumed to have a transparent bg */
:root {
    color-scheme: light dark;

    --text: black;
    --bg: white;

    --unofficial-watermark: url(https://www.w3.org/StyleSheets/TR/2016/logos/UD-watermark);

    --logo-bg: #1a5e9a;
    --logo-active-bg: #c00;
    --logo-text: white;

    --tocnav-normal-text: #707070;
    --tocnav-normal-bg: var(--bg);
    --tocnav-hover-text: var(--tocnav-normal-text);
    --tocnav-hover-bg: #f8f8f8;
    --tocnav-active-text: #c00;
    --tocnav-active-bg: var(--tocnav-normal-bg);

    --tocsidebar-text: var(--text);
    --tocsidebar-bg: #f7f8f9;
    --tocsidebar-shadow: rgba(0,0,0,.1);
    --tocsidebar-heading-text: hsla(203,20%,40%,.7);

    --toclink-text: var(--text);
    --toclink-underline: #3980b5;
    --toclink-visited-text: var(--toclink-text);
    --toclink-visited-underline: #054572;

    --heading-text: #005a9c;

    --hr-text: var(--text);

    --algo-border: #def;

    --del-text: red;
    --del-bg: transparent;
    --ins-text: #080;
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">RDF Surfaces Primer</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="profile-and-date"><span class="content">Living Document, <time class="dt-updated" datetime="2024-05-16">16 May 2024</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>Issue Tracking:
     <dd><a href="#issues-index">Inline In Spec</a>
     <dt class="editor">Editors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:patrick.hochstenbach@ugent.be">Patrick Hochstenbach</a> (<span class="p-org org"><a href="https://knows.idlab.ugent.be/team/">KNoWS</a></span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jos.deroo@ugent.be">Jos De Roo</a> (<span class="p-org org"><a href="https://knows.idlab.ugent.be/team/">KNoWS</a></span>)
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"> <a href="http://creativecommons.org/publicdomain/zero/1.0/" rel="license"><img alt="CC0" height="15" src="https://licensebuttons.net/p/zero/1.0/80x15.png" width="80"></a> To the extent possible under law, the editors have waived all copyright and related or neighboring rights to this work. In addition, as of 16 May 2024, the editors have made this specification available under the <a href="https://www.openwebfoundation.org/the-agreements/the-owf-1-0-agreements-granted-claims" rel="license">Open Web Foundation Agreement Version 1.0</a>, which is available at https://www.openwebfoundation.org/the-agreements/the-owf-1-0-agreements-granted-claims. Parts of this work may be from another specification document.  If so, those parts are instead covered by the license of that specification document. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This document specifies a Notation3 sublanguage to express RDF Surfaces: RDF with first-order logic semantics.</p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#Introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#document-conventions"><span class="secno">2</span> <span class="content">Document Conventions</span></a>
    <li>
     <a href="#Surface"><span class="secno">3</span> <span class="content">Surface</span></a>
     <ol class="toc">
      <li><a href="#PositiveSurface"><span class="secno">3.1</span> <span class="content">Default Positive Surface</span></a>
      <li><a href="#NegativeSurface"><span class="secno">3.2</span> <span class="content">Negative Surface</span></a>
      <li><a href="#answer-surface"><span class="secno">3.3</span> <span class="content">Answer Surface</span></a>
     </ol>
    <li><a href="#Examples"><span class="secno">4</span> <span class="content">Examples</span></a>
    <li><a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="Introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#Introduction"></a></h2>
   <p><img alt="Logo" src="images/logo.svg" style="float: left; margin: 5px;" width="100"></p>
   <p>This document presents a <a href="https://w3c.github.io/N3/spec/">Notation3</a> sublanguage
to express a revised RDF logic as envisioned by <a href="https://www.ihmc.us/groups/phayes/">Pat Hayes</a> in his 2009 ISWC Invited Talk: <a href="https://www.slideshare.net/PatHayes/blogic-iswc-2009-invited-talk">BLOGIC</a>.
RDF Surfaces logic can be thought as an implementation of <a href="https://en.wikipedia.org/wiki/Charles_Sanders_Peirce">Charles Sanders Peirce</a>'s <a href="http://www.jfsowa.com/peirce/ms514.htm">Existential Graphs</a> in RDF.</p>
   <p>An RDF surface is a kind of sheet of paper on which RDF graphs can be written.
All triples that are part of an RDF graph are then on this sheet of paper, including all <a data-link-type="biblio" href="#biblio-uri" title="Uniform Resource Identifier (URI): Generic Syntax">[URI]</a>s, literals, and <a href="https://www.w3.org/TR/rdf11-mt/#blank-nodes">Blank nodes</a>. 
A sheet of paper can contain more than one RDF graph. An RDF graph can’t be split
over multiple sheets of paper. However, one can <em>copy</em> an RDF graph from one sheet of 
paper to another sheet of paper.</p>
   <p><a href="https://www.w3.org/TR/rdf11-mt/#blank-nodes">Blank nodes</a> are special. They can’t
be copied to a new piece of paper. They can be thought of as scratches on a piece of 
paper. These scratches, or graffiti, are engraved into the piece of paper and can’t
be transferred. When copying data with blank nodes onto a new sheet of paper, 
new scratches need to be made.</p>
   <div class="example" id="example-f4b4150e">
    <a class="self-link" href="#example-f4b4150e"></a> An empty <em>positive surface</em> is a tautology. 
    <p><img alt="blank surface" src="images/positive_surface.svg" width="300"></p>
   </div>
   <div class="example" id="example-1e0582c1">
    <a class="self-link" href="#example-1e0582c1"></a> A <em>positive surface</em> with one triple <code>:Gent a :City</code>, which means <em>"Ghent is a city"</em>. 
    <p><img alt="blank surface" src="images/one_assertion.svg" width="300"></p>
   </div>
   <div class="example" id="example-d0eadebe">
    <a class="self-link" href="#example-d0eadebe"></a> A <em>positive surface</em> with one triple containing a blank node <code>[] a :City</code>, which
means <em>"There is something that is a city"</em>. 
    <p><img alt="blank surface" src="images/blank_node.svg" width="300"></p>
   </div>
   <div class="example" id="example-2624412b">
    <a class="self-link" href="#example-2624412b"></a> Two <em>positive surfaces</em>, one with the triple <code>[] a :City</code> (which means <em>"There is something that is a city"</em>),
and another one with the triple <code>[] a :Cat</code> (which means <em>"There is something that is a cat"</em>). 
    <p><img alt="blank surface" src="images/two_sheets.svg" width="600"></p>
    <p>One can copy these two RDF graphs to a new surface. This will contain two graphs with
two blank nodes:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->

<c- p>[]</c-> <c- b>a</c-> <c- p>:</c-><c- f>City</c-> <c- p>.</c->
<c- p>[]</c-> <c- b>a</c-> <c- p>:</c-><c- f>Cat</c-> <c- p>.</c->
</pre>
    <p>This means, "There is something that is a city AND There is something that is a cat."</p>
    <p><img alt="blank surface" src="images/city_cat.svg" width="300"></p>
   </div>
   <p><strong>Positive surface</strong></p>
   <p>The default surface on which RDF graphs are written is a <strong>positive surface</strong>.
For instance, the sheet of paper in Example 1 is an example of a <strong>positive surface</strong>.
(In this document, we use a paper with a black border as a positive surface.) Any
RDF triple written on this surface is interpreted as logical assertion (true). An empty 
sheet of positive paper is an empty claim and is treated as a logical tautology (true).</p>
   <p>When there is more than one RDF triple on the surface, it is a logical conjunction (AND).
If we interpret the sheets of paper with black borders in the examples above as
positive surfaces, then they express:</p>
   <ul>
    <li data-md>
     <p>Example 1: true.</p>
    <li data-md>
     <p>Example 2: It is true that: Ghent a City.</p>
    <li data-md>
     <p>Example 3: It is true that: ∃ x : x  a City.</p>
    <li data-md>
     <p>Example 4: It is true that: ∃ x , y : ( x a City ) AND ( y a Cat ).</p>
   </ul>
   <p>The <a href="https://www.w3.org/TR/rdf11-mt/#blank-nodes">blank nodes</a> need to be interpreted
as existential quantified variables on logical paper.</p>
   <p><strong>Negative surface</strong></p>
   <p>In the same way that a <em>positive surface</em> asserts a logical truth, a <strong>negative surface</strong> asserts a logical negation. The examples below will use a sheet of paper with a red border
as a negative surface.</p>
   <p>An empty <em>negative surface</em> on the default positive surface expresses a logical contradiction. When one
or more RDF graphs are written on a <em>negative</em> surface, they mean the negation of those RDF graphs.</p>
   <p>A blank node on a negative surface is interpreted as a universal quantified variable. The reason is
as follows:</p>
<pre>NOT(∃ x : P(x)) ⇔ ∀ x : NOT(P(x))
</pre>
   <div class="example" id="example-48a1bce7">
    <a class="self-link" href="#example-48a1bce7"></a> An empty <em>negative surface</em> (on a default positive surface) is a contradiction. 
    <p><img alt="blank surface" src="images/negative_surface.svg" width="300"></p>
   </div>
   <div class="example" id="example-e293fda3">
    <a class="self-link" href="#example-e293fda3"></a> A <em>negative surface</em> with one triple <code>:Ghent a :Cat</code> means <em>"It is not the case that Ghent is a cat"</em>. 
    <p><img alt="blank surface" src="images/city_not_cat.svg" width="300"></p>
   </div>
   <div class="example" id="example-412597b8">
    <a class="self-link" href="#example-412597b8"></a> A <em>negative surface</em> with one triple <code>[] a :City</code> means <em>"It is not the case that something is a city"</em> , or <em>"Nothing is a city"</em>. 
    <p><img alt="blank surface" src="images/nothing_a_city.svg" width="300"></p>
   </div>
   <p><strong>Propositional logic</strong></p>
   <p>With combinations of conjunction, by putting triples on a <em>positive surface</em>,
and negation, by putting triples on a <em>negative surface</em>, any <strong>compound 
truth-functional statement</strong> can be symbolized with positive and negative sheets
of paper:</p>
   <ul>
    <li data-md>
     <p>The logical conjunction <strong>AND</strong> is given by putting RDF triples on a <em>positive surface</em>.</p>
    <li data-md>
     <p>The logical negation <strong>NOT</strong> is given by putting RDF triples on a <em>negative surface</em>.</p>
    <li data-md>
     <p>The logical disjunction <strong>OR</strong> can be made by a combination of <em>positive</em> and <em>negative surfaces</em>:</p>
     <ul>
      <li data-md>
       <p><code>NOT ( P AND Q ) = NOT ( P ) OR NOT ( Q )</code></p>
      <li data-md>
       <p><code>NOT ( NOT ( P ) AND NOT ( Q ) ) = P OR Q</code></p>
     </ul>
    <li data-md>
     <p>The logical implication <strong>→</strong> can be made with a combination of <strong>AND</strong> and <strong>OR</strong>:</p>
     <ul>
      <li data-md>
       <p><code>P → Q = NOT ( P ) OR Q = NOT ( P AND NOT ( Q ) )</code></p>
     </ul>
   </ul>
   <div class="example" id="example-ffdd94a1">
    <a class="self-link" href="#example-ffdd94a1"></a> Propositional logic using <em>positive</em> and <em>negative</em> surfaces. 
    <p><img alt="blank surface" src="images/propositional_logic.svg" width="500"></p>
   </div>
   <p><strong>First-order logic</strong></p>
   <p>First-order logic can be added to the RDF surfaces by interpreting a blank node as
an existential quantified variable, and using the rule that a universal quantified variable can be made from an 
existential quantified variable by placing it in an enclosing negative surface:</p>
   <ul>
    <li data-md>
     <p>A blank node on a <em>positive surface</em> references an existential quantified variable.</p>
    <li data-md>
     <p>A blank node on a <em>negative surface</em> references a universal quantified variable.</p>
   </ul>
   <div class="example" id="example-a0c6b17c">
    <a class="self-link" href="#example-a0c6b17c"></a> First-order logic using <em>positive</em> and <em>negative</em> surfaces. 
    <p><img alt="blank surface" src="images/first_order_logic.svg" width="500"></p>
   </div>
   <p><strong>First-order logic in Notation3</strong></p>
   <p>RDF Surfaces provide a way to express the notion of <em>positive</em> and <em>negative surfaces</em> in Notation3 with help of the built-in <code>log:onNegativeSurface</code>. A <code>log:onNegativeSurface</code> is true when at least one of the nested triples in the object is false. A <em>negative</em> surface
can be constructed from a single <code>log:onNegativeSurface</code> built-in. A <em>positive</em> surface
can be constructed from double-nested <code>log:onNegativeSurface</code> built-ins.</p>
   <p>The subjects of both built-ins are the blank nodes that need to be put on the
surfaces (so that they become existential or universal quantified variables,
depending on the nesting level of the surface).</p>
   <div class="example" id="example-51ab59e2">
    <a class="self-link" href="#example-51ab59e2"></a> <em>"Something is a city"</em> in Notation3. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
  <c- p>(</c-><c- nn>_</c-><c- p>:</c-><c- f>X</c-><c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>City</c-> <c- p>.</c->
  <c- p>}</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <div class="example" id="example-58f68c9e">
    <a class="self-link" href="#example-58f68c9e"></a> <em>"Something is a city"</em> in Notation3, version 2: because the default surface 
is defined as a positive surface. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>City</c-> <c- p>.</c->
</pre>
   </div>
   <div class="example" id="example-ad27acea">
    <a class="self-link" href="#example-ad27acea"></a> <em>"Everything is a city"</em> in Notation3. Note that we can make universal or existential
quantified variables by placing blank nodes on an oddly- or evenly-nested surface. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
  <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>City</c-> <c- p>.</c->
  <c- p>}</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <div class="example" id="example-f6fbb6eb">
    <a class="self-link" href="#example-f6fbb6eb"></a> <em>"Nothing is a problem"</em> in Notation3. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Problem</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <div class="example" id="example-249e4055">
    <a class="self-link" href="#example-249e4055"></a> <em>"All cats are alive OR dead"</em> in Notation3. These are Schrödinger cats: logical OR can
mean alive, or dead, or both alive and dead. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Cat</c-> <c- p>.</c->

    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
        <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>is</c-> <c- p>:</c-><c- f>Alive</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->

    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
        <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>is</c-> <c- p>:</c-><c- f>Dead</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <div class="example" id="example-81410f2d">
    <a class="self-link" href="#example-81410f2d"></a> <em>"If any cat X is alive, then X says meow"</em> in Notation3. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Cat</c-> <c- p>;</c->
        <c- p>:</c-><c- f>is</c-> <c- p>:</c-><c- f>Alive</c-> <c- p>.</c->
    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
        <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>says</c-> <c- p>:</c-><c- f>Meow</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <p>A surface can be queried by providing a query surface with the <code>log:onNegativeAnswerSurface</code> built-in. The results of this query will be the final results of the reasoning engine
interpreting the surfaces.</p>
   <div class="example" id="example-1eeb967f">
    <a class="self-link" href="#example-1eeb967f"></a> 
    <p><code>:Ghent a :City</code> is a triple on the implicit positive surface. The two nested
negative surfaces express that for any subject <code>_:S</code> on the positive surface
that is a <code>:City</code>, it is implied that <code>_:S</code> if also a <code>:HumanCommunity</code>.</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>ex:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- nn>ex</c-><c- p>:</c-><c- f>Ghent</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>City</c-><c- p>.</c->

<c- c># Every city is a human community</c->
<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>S</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>S</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>City</c-> <c- p>.</c->
    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
        <c- nn>_</c-><c- p>:</c-><c- f>S</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>HumanCommunity</c-> <c- p>.</c->
    <c- p>}.</c->
<c- p>}.</c->

<c- c># Query</c->
<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>S</c-> <c- nn>_</c-><c- p>:</c-><c- f>C</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>S</c-> <c- b>a</c-> <c- nn>_</c-><c- p>:</c-><c- f>C</c-> <c- p>.</c->
    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeAnswerSurface</c-> <c- p>{</c->
      <c- nn>_</c-><c- p>:</c-><c- f>S</c-> <c- b>a</c-> <c- nn>_</c-><c- p>:</c-><c- f>C</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
    <p>When this surface is executed by a reasoner, the result would be:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>ex:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->

<c- nn>ex</c-><c- p>:</c-><c- f>Ghent</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>City</c-> <c- p>.</c->
<c- nn>ex</c-><c- p>:</c-><c- f>Ghent</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>HumanCommunity</c-> <c- p>.</c->
</pre>
   </div>
   <p>More test cases can be found at <a href="https://github.com/eyereasoner/rdfsurfaces-tests">https://github.com/eyereasoner/rdfsurfaces-tests</a>.</p>
   <h2 class="heading settled" data-level="2" id="document-conventions"><span class="secno">2. </span><span class="content">Document Conventions</span><a class="self-link" href="#document-conventions"></a></h2>
   <p>Within this document, the following namespace prefix bindings to <a data-link-type="biblio" href="#biblio-uri" title="Uniform Resource Identifier (URI): Generic Syntax">[URI]</a>s are used:</p>
   <table>
    <thead>
     <tr>
      <th>Prefix 
      <th>Namespace 
    <tbody>
     <tr>
      <td>ex 
      <td>http://example.org/ns# 
     <tr>
      <td>rdf 
      <td>http://www.w3.org/1999/02/22-rdf-syntax-ns# 
     <tr>
      <td>log 
      <td>http://www.w3.org/2000/10/swap/log# 
     <tr>
      <td>list 
      <td>http://www.w3.org/2000/10/swap/list# 
     <tr>
      <td>string 
      <td>http://www.w3.org/2000/10/swap/string# 
   </table>
   <h2 class="heading settled" data-level="3" id="Surface"><span class="secno">3. </span><span class="content">Surface</span><a class="self-link" href="#Surface"></a></h2>
   <p>Surfaces are written as triples where the <code>subject</code> is a list of zero or more blank nodes.
The <code>object</code> is a graph term or a boolean literal. The blank nodes in the 
subject list are treated as marks on the object RDF graph. The <code>predicate</code> specifies 
the kind of surface. Any kind of surface may be used, but the following built-ins have 
special semantics:</p>
   <table>
    <thead>
     <tr>
      <td>built-in 
      <td>semantics 
    <tbody>
     <tr>
      <td><code>log:onNegativeSurface</code> 
      <td>Negative surface claim that any RDF graph in the object position is false. 
   </table>
   <p>A surface can contain zero or more other surfaces. These contained surfaces are "nested".
Nested surfaces can share the same <span>[URIs]</span> and literals (by copying the data), 
but can’t share blank nodes. Any blank nodes that are written inside a surface 
(not as subject of an RDF Surface) are to be interpreted as <em>coreferences</em> to the
blank node graffiti defined on a parent RDF Surface. If no such parent RDF Surface
exists, then it is assumed that the blank node is a coreference to an implicitly declared
blank node graffiti on the default positive surface.</p>
   <div class="example" id="example-4b1c84e2">
    <a class="self-link" href="#example-4b1c84e2"></a> An <code>ex:myFirstSurface</code> is created which contains two triples and a nested surface. The
blank node in <code>_:X a ex:OtherStatement</code> is a co-reference to the <code>_:X</code> graffiti on <code>ex:myFirstSurface</code>. 
    <p>In the nested surface <code>ex:mySecondSurface</code>, the blank node in <code>_:X ex:is true</code> is also a
reference to the <code>_:X</code> blank node graffiti on <code>ex:myFirstSurface</code>.</p>
    <p>The blank node <code>_:Y</code> co-reference in <code>ex:mySecondSurface</code> doesn’t refer to a parent blank
node graffiti. It is assumed that the default positive surface contains that graffiti.</p>
    <p>In the nested surface <code>ex:myThirdSurface</code>, the blank node in <code>_:X ex:is false</code> is a
co-reference to the <code>_:X</code> blank node graffiti on <code>ex:myThirdSurface</code>. The new graffiti
shields the <code>_:X</code> that was defined on <code>ex:myFirstSurface</code>: the scope of <code>_:X</code> co-references is now limited to <code>ex:myThirdSurface</code>.</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>ex:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->

<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>ex</c-><c- p>:</c-><c- f>myFirstSurface</c-> <c- p>{</c->

    <c- nn>ex</c-><c- p>:</c-><c- f>Statement1</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>NiceStatement</c-> <c- p>.</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>OtherStatement</c-> <c- p>.</c->
     
    <c- p>()</c-> <c- nn>ex</c-><c- p>:</c-><c- f>mySecondSurface</c-> <c- p>{</c->
        <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- nn>ex</c-><c- p>:</c-><c- f>is</c-> <c- l>true</c-> <c- p>.</c-> 
        <c- nn>_</c-><c- p>:</c-><c- f>Y</c-> <c- b>a</c-> <c- nn>ex</c-><c- p>:</c-><c- f>AnotherStatement</c-> <c- p>.</c->

        <c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>ex</c-><c- p>:</c-><c- f>myThirdSurface</c-> <c- p>{</c->
            <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- nn>ex</c-><c- p>:</c-><c- f>is</c-> <c- l>false</c-> <c- p>.</c->
        <c- p>}</c-> <c- p>.</c->
    <c- p>}.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <h3 class="heading settled" data-level="3.1" id="PositiveSurface"><span class="secno">3.1. </span><span class="content">Default Positive Surface</span><a class="self-link" href="#PositiveSurface"></a></h3>
   <p>The default positive surface claims that any RDF Graph that is placed on it is true.
This is the current interpretation of <a href="https://www.w3.org/TR/rdf11-mt/">RDF Semantics</a>.
When no surfaces are provided in an RDF document, the implicit default positive surface is assumed.</p>
   <div class="example" id="example-e5dc235d">
    <a class="self-link" href="#example-e5dc235d"></a> The two surfaces below are equal (because a double negation of a statement is the same as
claiming the statement is true, i.e., positive): 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
</pre>
    <p>and</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
  <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
  <c- p>}</c-> <c- p>.</c->
<c- p>}</c->
</pre>
   </div>
   <p>The semantics of the default positive surface are interpreted as a logical truth:</p>
   <ul>
    <li data-md>
     <p>An empty RDF graph on the default positive surface is a tautology.</p>
    <li data-md>
     <p>When a triple is placed on the default positive surface, the surface is valid when the triple is true.</p>
    <li data-md>
     <p>When two or more triples are placed on the default positive surface, the surface is valid when the
  logical conjunction of all triples is true.</p>
   </ul>
   <div class="example" id="example-dcc6b417">
    <a class="self-link" href="#example-dcc6b417"></a> The surface below should be interpreted as: "Alice is a person and knows Bob, and Bob is a 
person and knows Alice". 
    <p>As a logical statement:</p>
<pre>:Alice a :Person AND 
:Alice :knows :Bob AND (
  :Bob a :Person AND 
  :Bob :knows :Alice
)
</pre>
    <p>As RDF Surface:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
<c- p>:</c-><c- f>Alice</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Bob</c-> <c- p>.</c->

<c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
  <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- p>:</c-><c- f>Bob</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- p>:</c-><c- f>Bob</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Alice</c-> <c- p>.</c->
  <c- p>}</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <p>When a blank node is marked on an evenly-nested negative surface, it is interpreted
as an existential quantified variable in the scope of the nested surface.</p>
   <div class="example" id="example-bede3423">
    <a class="self-link" href="#example-bede3423"></a> The surface below should be interpreted as: "There is a person that knows Alice". 
    <p>As a logical statement:</p>
<pre>∃ _:X : _:X a :Person AND _:X :knows :Alice
</pre>
    <p>Or, rewritten using NOT:</p>
<pre>NOT ( NOT ( ∃ _:X : _:X a :Person AND _:X :knows :Alice ) )
</pre>
    <p>We make use of the logical rule that a negation of a negation cancels, and creates a positive statement.</p>
    <p>As RDF Surface:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
  <c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Alice</c-> <c- p>.</c->
  <c- p>}</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <p>In RDF Surfaces, a default positive surface is implicitly assumed for each
RDF document. On this default positive surface, all existential variables
are implicitly quantified. Example 20 can be rewritten as:</p>
   <div class="example" id="example-891ff7ba">
    <a class="self-link" href="#example-891ff7ba"></a> A default positive surface with an implicit <code>_:X</code> existential variable. 
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
<c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Alice</c-> <c- p>.</c->
</pre>
   </div>
   <p>When a blank node is marked on an oddly-nested negative surface, it is interpreted
as a universal quantified variable in the scope of the nested surface.</p>
   <div class="example" id="example-5bb77dee">
    <a class="self-link" href="#example-5bb77dee"></a> The surface below should be interpreted as: "Everything is a person and knows Alice". 
    <p>As a logical statement:</p>
<pre>∀ _:X : _:X a :Person AND _:X :knows :Alice
</pre>
    <p>Or, rewritten using NOT:</p>
<pre>NOT( ∃ _:X : NOT ( _:X a :Person AND _:X :knows :Alice ) )
</pre>
    <p>We make here use of the logical rule that <code>NOT ( ∃ _:X ... )</code> is equal to <code>∀ _:X NOT ( ... )</code>.</p>
    <p>As RDF Surface:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>(</c-><c- nn>_</c-><c- p>:</c-><c- f>X</c-><c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
  <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Alice</c-> <c- p>.</c->
  <c- p>}</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <h3 class="heading settled" data-level="3.2" id="NegativeSurface"><span class="secno">3.2. </span><span class="content">Negative Surface</span><a class="self-link" href="#NegativeSurface"></a></h3>
   <p>A negative surface claims that any RDF Graph that is placed on it is false. The interpretation of the
negative surface is the negation of the RDF Graph that is placed on it.</p>
   <p>The semantics of a negative surface are interpreted as a logical falsehood:</p>
   <ul>
    <li data-md>
     <p>An empty negative surface is a contradiction.</p>
    <li data-md>
     <p>When a triple is placed on a negative surface, the surface is valid when the triple is false.</p>
    <li data-md>
     <p>When two or more triples are placed on a negative surface, the surface is valid when the logical
  conjunction of all triples is false.</p>
   </ul>
   <div class="example" id="example-9381ce35">
    <a class="self-link" href="#example-9381ce35"></a> The surface below should be interpreted as: "Alice is a person and knows Bob, and Bob is
not a person and does not know Alice". 
    <p>As a logical statement:</p>
<pre>:Alice a :Person AND 
:Alice :knows :Bob AND NOT (
  :Bob a :Person AND 
  :Bob :knows :Alice
)
</pre>
    <p>As RDF Surface:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
<c- p>:</c-><c- f>Alice</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Bob</c-> <c- p>.</c->

<c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- p>:</c-><c- f>Bob</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- p>:</c-><c- f>Bob</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Alice</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <p>When a blank node is marked on a negative surface, it should be interpreted as an existential
quantified variable in the scope of the negative surface.</p>
   <div class="example" id="example-f94a56e6">
    <a class="self-link" href="#example-f94a56e6"></a> The surface below should be interpreted as "Alice is a Person and knows Bob, 
and there isn’t a person that knows Alice". 
    <p>As a logical statement:</p>
<pre>:Alice a :Person AND 
:Alice :knows :Bob AND NOT (
  ∃ _:X :
     _:X a :Person AND 
     _:X :knows :Alice
)
</pre>
    <p>which is the same as:</p>
<pre>:Alice a :Person AND 
:Alice :knows :Bob AND 
  ( ∀ _:X :
     NOT (
      _:X a :Person AND 
      _:X :knows :Alice
    )
  ) 
)
</pre>
    <p>As RDF Surface:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
<c- p>:</c-><c- f>Alice</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Bob</c-> <c- p>.</c->

<c- p>(</c-> <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>)</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- nn>_</c-><c- p>:</c-><c- f>X</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Alice</c-> <c- p>.</c->
<c- p>}</c-> <c- p>.</c->
</pre>
   </div>
   <p>When two or more negative surfaces are nested in a parent negative surface, it should
be interpreted as a logical disjunction.</p>
   <div class="example" id="example-9b36f666">
    <a class="self-link" href="#example-9b36f666"></a> The surface below should be interpreted as "Alice is a Person OR Bob is a Person
OR Charly is a person". 
    <p>As a logical statement:</p>
<pre>:Alice a :Person OR
:Bob a :Person OR
:Charly a :Person 
</pre>
    <p>which is the same as:</p>
<pre>NOT (
  NOT ( :Alice a :Person ) AND
  NOT ( :Bob a :Person ) AND
  NOT ( :Charly a :Person )
)
</pre>
    <p>As RDF Surface:</p>
<pre class="highlight"><c- k>@prefix</c-> <c- nn>:</c-> <c- g>&lt;http://example.org/ns#></c-> <c- p>.</c->
<c- k>@prefix</c-> <c- nn>log:</c-> <c- g>&lt;http://www.w3.org/2000/10/swap/log#></c-> <c- p>.</c->

<c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
<c- p>:</c-><c- f>Alice</c-> <c- p>:</c-><c- f>knows</c-> <c- p>:</c-><c- f>Bob</c-> <c- p>.</c->

<c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
      <c- p>:</c-><c- f>Alice</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->

    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
      <c- p>:</c-><c- f>Bob</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->

    <c- p>()</c-> <c- nn>log</c-><c- p>:</c-><c- f>onNegativeSurface</c-> <c- p>{</c->
      <c- p>:</c-><c- f>Charly</c-> <c- b>a</c-> <c- p>:</c-><c- f>Person</c-> <c- p>.</c->
    <c- p>}</c-> <c- p>.</c->
<c- p>}.</c->
</pre>
   </div>
   <p>Other logical truth functions can be built with combinations of <code>AND</code> and <code>NOT</code>, such as the following:</p>
   <ul>
    <li data-md>
     <p>Disjunction: <code>P ∨ Q</code> : <code>NOT ( NOT ( P ) AND NOT ( Q ) )</code></p>
    <li data-md>
     <p>Material implication <code>P → Q</code> : <code>NOT ( P AND NOT Q )</code> .</p>
    <li data-md>
     <p>Converse implication <code>P ← Q</code> : <code>NOT ( NOT ( P ) AND Q )</code> .</p>
    <li data-md>
     <p>Biconditional <code>(P → Q) ∧ (Q → P)</code> : <code>NOT ( P AND NOT Q ) AND NOT ( Q AND NOT P )</code></p>
   </ul>
   <p>Negative surfaces should also follow the law of double negation elimination: <code>P = NOT ( NOT ( P ) )</code>.</p>
   <h3 class="heading settled" data-level="3.3" id="answer-surface"><span class="secno">3.3. </span><span class="content">Answer Surface</span><a class="self-link" href="#answer-surface"></a></h3>
   <p class="issue" id="issue-b7b1e314"><a class="self-link" href="#issue-b7b1e314"></a> TODO</p>
   <h2 class="heading settled" data-level="4" id="Examples"><span class="secno">4. </span><span class="content">Examples</span><a class="self-link" href="#Examples"></a></h2>
   <p>As an illustration of RDF Surface, we offer a graph traversal example. Example 26 provides information
about French roads where some roads are blocked due to road works. In this example, we use the predicate <code>:oneway</code> to indicate that a connection is possible from one city to another. To deny this connection,
we use a negative surface.</p>
   <div class="example" id="example-6cb4c32f">
    <a class="self-link" href="#example-6cb4c32f"></a> Possible connections between the French cities including a blocked connection between Chartes and Lemans. 
<pre>@prefix : &lt;urn:example:> .

:Paris :oneway :Orleans .
:Paris :oneway :Chartres .
:Paris :oneway :Amiens .
:Orleans :oneway :Blois .
:Orleans :oneway :Bourges .
:Blois :oneway :Tours .
:Lemans :oneway :Angers .
:Lemans :oneway :Tours .
:Angers :oneway :Nantes .

# blocking some roads
() log:onNegativeSurface {
    :Chartres :oneway :Lemans .
}.
</pre>
   </div>
   <p>Next, we would like to provide the logic of a "path" through the French road system. We would
like to share an RDF document that states: "When there is a <code>:oneway</code> from one city to another
city, then there is a <code>:path</code> from the first to the second". Or, expressed in a symbolic form:</p>
<pre>∀ _:A, _:B : ( _:A :oneway _:B ) IMPLIES ( _:A :path _:B )
</pre>
   <p>These paths are transitive. When there is a path from A to B and a path from B to C, there
is a path from A to C. Or, expressed in a symbolic form:</p>
<pre>∀ _:A, _:B, _:C : ( ( _:A :path _:B ) AND ( _:B :path _:C ) ) IMPLIES ( _:A :path _:C )
</pre>
   <p>Using the patterns for <code>IMPLIES</code> from the previous sections these logical formulas can be written
as RDF Surfaces in Example 27.</p>
   <div class="example" id="example-ce8fdd55">
    <a class="self-link" href="#example-ce8fdd55"></a> The RDF Surfaces of a path algorithm. 
<pre>@prefix log: &lt;http://www.w3.org/2000/10/swap/log#> .
@prefix : &lt;urn:example:> .

# oneway subproperty of path
( _:A _:B ) log:onNegativeSurface {
    _:A :oneway _:B .
    () log:onNegativeSurface {
        _:A :path _:B .
    } .
} .

# path transitive property
( _:A _:B _:C ) log:onNegativeSurface {
    _:A :path _:B .
    _:B :path _:C .
    () log:onNegativeSurface {
        _:A :path _:C .
    } .
} .
</pre>
   </div>
   <p>Based on the RDF documents in Example 26 and 27, we can construct a query to find out which paths are
possible that end in Nantes. For this, we use a specialized answer surface that expresses: "If there is
some path from A to Nantes, then this is an answer". This is expressed as RDF Surfaces in Example 28.</p>
   <div class="example" id="example-a67d5fce">
    <a class="self-link" href="#example-a67d5fce"></a> Query for a possible path from A to Nantes. 
<pre>@prefix log: &lt;http://www.w3.org/2000/10/swap/log#> .
@prefix : &lt;urn:example:> .

(_:A) log:onNegativeSurface {
    _:A :path :nantes .
    () log:onNegativeAnswerSurface {
        _:A :path :nantes .
        :test :is :true .
    } .
} .
</pre>
   </div>
   <p>Combining the RDF documents from Examples 26, 27, and 28, an RDF Surfaces reasoner should give as answers:</p>
<pre>:Angers :path :Nantes .
:Lemans :path :Nantes .
</pre>
   <p>If we remove the negative surface around <code>:Chartes :oneway :Lemans</code>, an RDF Surfaces reasoner
should provide more answers:</p>
<pre>:Angers :path :Nantes .
:Lemans :path :Nantes .
:Chartres :path :Nantes .
:Paris :path :Nantes .
</pre>
   <p>For more RDF Surfaces examples, see:</p>
   <ul>
    <li data-md>
     <p><a href="https://github.com/eyereasoner/rdfsurfaces-tests">https://github.com/eyereasoner/rdfsurfaces-tests</a></p>
   </ul>
  </main>
  <div data-fill-with="conformance">
   <h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
   <p> Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology.
            The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL”
            in the normative parts of this document
            are to be interpreted as described in RFC 2119.
            However, for readability,
            these words do not appear in all uppercase letters in this specification. </p>
   <p> All of the text of this specification is normative
            except sections explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119" title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</a> </p>
   <p> Examples in this specification are introduced with the words “for example”
            or are set apart from the normative text with <code>class="example"</code>, like this: </p>
   <div class="example" id="example-example"><a class="self-link" href="#example-example"></a> This is an example of an informative example. </div>
   <p> Informative notes begin with the word “Note”
            and are set apart from the normative text with <code>class="note"</code>, like this: </p>
   <p class="note" role="note"> Note, this is an informative note.</p>
  </div>
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    /* Create the sidebar toggle in JS; it shouldn't exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn't standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don't have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we're at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can't find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-rfc2119">[RFC2119]
   <dd>S. Bradner. <a href="https://datatracker.ietf.org/doc/html/rfc2119"><cite>Key words for use in RFCs to Indicate Requirement Levels</cite></a>. March 1997. Best Current Practice. URL: <a href="https://datatracker.ietf.org/doc/html/rfc2119">https://datatracker.ietf.org/doc/html/rfc2119</a>
  </dl>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-uri">[URI]
   <dd>T. Berners-Lee; R. Fielding; L. Masinter. <a href="https://www.rfc-editor.org/rfc/rfc3986"><cite>Uniform Resource Identifier (URI): Generic Syntax</cite></a>. January 2005. Internet Standard. URL: <a href="https://www.rfc-editor.org/rfc/rfc3986">https://www.rfc-editor.org/rfc/rfc3986</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> TODO <a class="issue-return" href="#issue-b7b1e314" title="Jump to section">↵</a></div>
  </div>