static <A extends Concept> A findBackRef(Concept c, Class<A> type) {
  for (Concept.Ref r : c.backRefs)
    if (instanceOf(r.concept(), type))
      ret (A) r.concept();
  null;
}

static <A extends Concept> A findBackRef(Class<A> type, Concept c) {
  ret findBackRef(c, type);
}