static int conceptGraphSize(Concept c) {
  new IdentityHashMap seen;
  conceptGraphSize_impl(c, seen);
  ret l(seen);
}

static void conceptGraphSize_impl(Concept c, IdentityHashMap<Concept, Concept> seen) {
  if (c == null || seen.containsKey(c)) ret;
  seen.put(c, c);
  for (Concept.Ref ref : c._refs())
    conceptGraphSize_impl(ref!, seen);
}