static <A> L<A> keysWithValueContainingSubstring(Map<A, S> map, S s) {
  new L<A> l;
  for (A key : keys(map))
    if (contains(map.get(key), s))
      l.add(key);
  ret l;
}

static <A> L<A> keysWithValueContainingSubstring(S s, Map<A, S> map) {
  ret keysWithValueContainingSubstring(map, s);
}