static <A, B> L<A> keysWithValueThat(IPred<B> pred, Map<A, B> map) {
  if (map == null) null;
  new L<A> l;
  for (Map.Entry<A, B> e : map.entrySet())
    if (pred.get(e.getValue()))
      l.add(e.getKey());
  ret l;
}

static <A, B> L<A> keysWithValueThat(Map<A, B> map, IPred<B> pred) {
  ret keysWithValueThat(pred, map);
}

Began life as a copy of #1008062