Add extend for lists

To insert the items of the first given list at the end of the second given list.

Fixes #238.

infra.bs

@@ -799,6 +799,22 @@ out-of-bounds, except when used with <a for=list>exists</a>.
 <p>To <dfn export for=list>append</dfn> to a <a>list</a> that is not an <a>ordered set</a> is to
 add the given <a for=list>item</a> to the end of the list.
 
+<p>To <dfn export for=list>extend</dfn> a <a>list</a> |A| with a <a>list</a> |B|,
+<a for=list>for each</a> |item| of |B|, <a for=list>append</a> |item| to |A|.
+
+<div class=example id=example-list-extend>
+ <ol>
+  <li><p>Let |ghostbusters| be « "<code>Erin Gilbert</code>", "<code>Abby Yates</code>" ».
+
+  <li><p><a for=list>Extend</a> |ghostbusters| with « "<code>Jillian Holtzmann</code>",
+  "<code>Patty Tolan</code>" ».
+
+  <li><p><a>Assert</a>: |ghostbusters|'s <a for=list>size</a> is 4.
+
+  <li><p><a>Assert</a>: |ghostbusters|[2] is "<code>Jillian Holtzmann</code>".
+ </ol>
+</div>
+
 <p>To <dfn export for=list>prepend</dfn> to a <a>list</a> that is not an <a>ordered set</a> is to
 add the given <a for=list>item</a> to the beginning of the list.