// f : Concept -> bool
static Concept conceptGraphSearch(Concept c, O f) {
  ret conceptGraphSearch_impl(c, f, new IdentityHashMap);
}

static Concept conceptGraphSearch_impl(Concept c, O f, IdentityHashMap<Concept, Concept> seen) {
  if (seen.containsKey(c)) null;
  if (callPred(f, c)) ret c;
  seen.put(c, c);
  
  for (S field : conceptFields(c)) {
    O val = cget(c, field);
    if (val instanceof Concept) {
      Concept d = conceptGraphSearch_impl(val/Concept, f, seen);
      if (d != null) ret d;
    }
  }
  null;
}