// returns true if any changes were made (deletions or additions)
static bool post_correctThird(S a, S b, S c) {
  bool found = false, changes = false;
  for (TripleWeb w : ai_triples(a, b, "$X")) {
    if (eqic(w.c, c)) found = true;
    else { ai_invalidateTriple(w); changes = true; }
  }
  if (!found) {
    post(a, b, c); changes = true;
  }
  ret changes;
}