Add set extend operation

"List extend" didn't really work since it used "list append", which doesn't properly handle duplicates.

infra.bs

@@ -1542,8 +1542,8 @@ list multiple assignment syntax is used.
 <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|.
+<p>To <dfn export for=list>extend</dfn> a <a>list</a> that is not an <a>ordered set</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>
@@ -1709,6 +1709,9 @@ ordered sets; implementations can optimize based on the fact that the order is n
 the given <a for=set>item</a>, then do nothing; otherwise, perform the normal <a>list</a>
 <a for=list>append</a> operation.
 
+<p>To <dfn export for=set>extend</dfn> an [=ordered set=] |A| with a [=list=] |B|, [=list/for each=]
+|item| of |B|, [=set/append=] |item| to |A|.
+
 <p>To <dfn export for=set>prepend</dfn> to an <a>ordered set</a>: if the set
 <a for=list>contains</a> the given <a for=set>item</a>, then do nothing; otherwise, perform the
 normal <a>list</a> <a for=list>prepend</a> operation.