static Rat ai_relationSymmetryCount(S rel) {
  L<TripleWeb> l = triplesWithB(rel);
  new Set<Pair<S>> pairs;
  int i = 0, n = 0;
  for (TripleWeb w : l)
    pairs.add(stdCasePair(w.a, w.b));
  for (TripleWeb w : l) {
    ++n;
    if (pairs.contains(stdCasePair(w.b, w.a)))
      ++i;
  }
  print("l(l)=" + l(l) + ", i=" + i +", n=" + n);
  ret rat_noSimplify(i, n);
}