static <A, B> L<A> keysWithValue(Map<A, B> map, B value) {
  new L<A> l;
  for (A key : keys(map))
    if (eq(map.get(key), value))
      l.add(key);
  ret l;
}

static <A, B> L<A> keysWithValue(B value, Map<A, B> map) {
  ret keysWithValue(map, value);
}