// tag = tag name
// newTag = full new contents including outer tag
// replaces only one occurrence
// returns pair(new text, extracted tag)
static Pair<S> hextracttag(S html, S tag) {
  L<S> tok = htmlcoarsetok(html);
  L<L<S>> tags = findContainerTag(tok, tag);
  if (empty(tags)) ret pair(html, null);
  L<S> theTag = first(tags);
  L<S> actualTag = subList(theTag, 1, l(theTag)-1);
  S s = join(actualTag);
  ret pair(join(replaceSubList(cloneList(tok), actualTag, emptyList())), s);
}