static L<Pair<PTElement>> ai_matchParseTrees(PTElement a, PTElement b) {
  new L<Pair<PTElement>> out;
  ai_matchParseTrees_collect(a, b, out);
  ret out;
}

svoid ai_matchParseTrees_collect(PTElement a, PTElement b, L<Pair<PTElement>> out) {
  PTElement a2 = ai_parseTree_descendToComposite(a);
  PTElement b2 = ai_parseTree_descendToComposite(b);
  
  PTElement aa = or(a2, a), bb = or(b2, b);
  
  if (a2 != null && b2 != null) {
    ai_matchParseTrees_collect(first(a2.children), first(b2.children), out);
    ai_matchParseTrees_collect(second(a2.children), second(b2.children), out);
  } else
    addPair(out, aa, bb);
}