static L<Pair<S>> ai_allSubtypeRelations() {
  new L<Pair<S>> l;
  for (WebRelation r : indexedRelations("is")) {
    S a = web_text(r.a), b = web_text(r.b);
    if (ai_isAOrAn(a) && ai_isAOrAn(b))
      l.add(pair(a, b));
  }
  ret l;
}