diff --git a/doc/guide.md b/doc/guide.md
index 51fa0beb..81443dcc 100644
--- a/doc/guide.md
+++ b/doc/guide.md
@@ -269,6 +269,36 @@ path"$0
 
 `intercalate` is defined in `lib/string.jac`.
 
+#### Example
+
+Suppose we want to mimic some functionality of `sed` - we'd like to replace
+some regular expression with a string (no capture groups, only first replacement
+per line)
+
+```
+@include'prelude/fn.jac'
+
+fn replace1(re, str, line) :=
+  let 
+    val insert := \line. \str. \ixes.
+      take (ixes->1) line + str + drop (ixes->2) line
+  in option line (insert line str) (match line re) end;
+```
+
+Then we could trim whitespace from a file with
+
+```
+@include'lib/sed.jac'
+
+(replace1 /\s+$/ '')"$0
+```
+
+Jacinda does not modify files in-place so one would need to use [sponge](https://joeyh.name/code/moreutils/) perhaps:
+
+```
+ja run trimwhitespace.jac -i FILE | sponge FILE
+```
+
 #### Parting Shots
 
 ```
diff --git a/doc/guide.pdf b/doc/guide.pdf
index 4f4b9616..b375d734 100644
Binary files a/doc/guide.pdf and b/doc/guide.pdf differ
diff --git a/docs/index.html b/docs/index.html
index 3baf75b1..32d37e5d 100644
--- a/docs/index.html
+++ b/docs/index.html
@@ -362,6 +362,24 @@ <h4 id="file-includes">File Includes</h4>
 path&quot;$0</code></pre>
 <p><code>intercalate</code> is defined in
 <code>lib/string.jac</code>.</p>
+<h4 id="example">Example</h4>
+<p>Suppose we want to mimic some functionality of <code>sed</code> -
+we’d like to replace some regular expression with a string (no capture
+groups, only first replacement per line)</p>
+<pre><code>@include&#39;prelude/fn.jac&#39;
+
+fn replace1(re, str, line) :=
+  let 
+    val insert := \line. \str. \ixes.
+      take (ixes-&gt;1) line + str + drop (ixes-&gt;2) line
+  in option line (insert line str) (match line re) end;</code></pre>
+<p>Then we could trim whitespace from a file with</p>
+<pre><code>@include&#39;lib/sed.jac&#39;
+
+(replace1 /\s+$/ &#39;&#39;)&quot;$0</code></pre>
+<p>Jacinda does not modify files in-place so one would need to use <a
+href="https://joeyh.name/code/moreutils/">sponge</a> perhaps:</p>
+<pre><code>ja run trimwhitespace.jac -i FILE | sponge FILE</code></pre>
 <h4 id="parting-shots">Parting Shots</h4>
 <pre><code>or := [(||)|#f x]
 
diff --git a/examples/trimwhitespace.jac b/examples/trimwhitespace.jac
new file mode 100644
index 00000000..3077c62e
--- /dev/null
+++ b/examples/trimwhitespace.jac
@@ -0,0 +1,3 @@
+@include'lib/sed.jac'
+
+(replace1 /\s+$/ '')"$0
diff --git a/lib/sed.jac b/lib/sed.jac
new file mode 100644
index 00000000..2b7cff30
--- /dev/null
+++ b/lib/sed.jac
@@ -0,0 +1,8 @@
+@include'prelude/fn.jac'
+
+{. example: (replace /^var/ 'export const')"$0
+fn replace1(re, str, line) :=
+  let 
+    val insert := \line. \str. \ixes.
+      take (ixes->1) line + str + drop (ixes->2) line
+  in option line (insert line str) (match line re) end;