// tag = tag name
// newTag = full new contents including outer tag
// replaces only one occurrence
static S hreplacetag(S html, S tag, S newTag) {
  L<S> tok = htmlcoarsetok(html);
  L<L<S>> tags = findContainerTag(tok, tag);
  if (empty(tags)) ret html;
  L<S> theTag = first(tags);
  L<S> actualTag = subList(theTag, 1, l(theTag)-1);
  ret join(replaceSubList(cloneList(tok), actualTag, litlist(newTag)));
}