static L<Pair<O>> ai_matchParseTrees_v2(PTElement a, PTElement b) {
  new L<Pair<O>> out;
  ai_matchParseTrees_v2_collect(a, b, out);
  ret out;
}

svoid ai_matchParseTrees_v2_collect(PTElement a, PTElement b, L<Pair<O>> 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) {
    // The actual match is between the productions.
    // We add the tree nodes too for reference.
    addPair(out,
      pairWithToStringFromA(a2/ChoosePart.reconstruction.production, a2),
      pairWithToStringFromA(b2/ChoosePart.reconstruction.production, b2));
    ai_matchParseTrees_v2_collect(first(a2.children), first(b2.children), out);
    ai_matchParseTrees_v2_collect(second(a2.children), second(b2.children), out);
  } else
    addPair(out, aa, bb);
}