static void ai_spec_parseArrowTriple(S s) {
  new Matches m;
  T3<S> t = ai_parseArrowTriple(s);
  if (t == null) ret;
  ai_postTriple(s, "is a triple with left part", t.a);
  ai_postTriple(s, "is a triple with middle part", t.b);
  ai_postTriple(s, "is a triple with right part", t.c);
}