static <A extends Concept> L<A> findBackRefs(Collection<? extends Concept> concepts, Class<A> type) {
  new IdentityHashMap<A, Bool> l;
  for (Concept c : concepts)
    if (c.backRefs != null) for (Concept.Ref r : c.backRefs)
      if (instanceOf(r.concept(), type))
        l.put((A) r.concept(), true);
  ret asList(keys(l));
}

// TODO: sort by ID?
static <A extends Concept> L<A> findBackRefs(Concept c, Class<A> type) {
  new IdentityHashMap<A, Bool> l;
  if (c.backRefs != null) for (Concept.Ref r : c.backRefs)
    if (instanceOf(r.concept(), type))
      l.put((A) r.concept(), true);
  ret asList(keys(l));
}
  
static <A extends Concept> L<A> findBackRefs(Class<A> type, Concept c) {
  ret findBackRefs(c, type);
}