Empty Element

The empty element, contrary to the `Marker`, is a truly
empty, thin element that does nothing. It can be used
for bookkeeping, e.g., as the empty result of a search
or as an indicator for an invalid element.

For all other purposes, use `Marker`.

Renamed from `Null` because `Null`/`null`/`None`/`none`
are reserved keywords in the file formats and programming
languages we support in PALS.

Author: ax3l

source/element-kinds.md

@@ -386,18 +386,18 @@ Element parameter groups associated with this element kind are:
 - [**TrackingP**](#s:tracking.params): Tracking parameters.
 
 %---------------------------------------------------------------------------------------------------
-(s:nullele)=
-## NullEle Element
+(s:empy)=
+## Empty Element
 
 Placeholder element used for bookkeeping when constructing the expanded lattice.
 This element is empty, zero length and does nothing.
 This element can be used as a [`base_item`](#s:placement) element for [superpositions](#s:superposition). 
 Additionally, this element can be used, for example, to denote an invalid element in the internal
 structures defined by a program.
 
-`NullEle` elements present in a lattice file will, as a part of lattice expansion, be removed.
-That is, `NullEle` elements will never be present in the final expanded lattice and
-tracking through a `NullEle` will never be needed.
+`Empty` elements present in a lattice file will, as a part of lattice expansion, be removed.
+That is, `Empty` elements will never be present in the final expanded lattice and
+tracking through a `Empty` will never be needed.
 
 This element does not have any associated parameter groups.