static <A, B> A firstKeyWithValue(Map<A, B> map, B value) {
  if (map != null) for (Map.Entry<A, B> e : map.entrySet())
    if (eq(e.getValue(), value))
      ret e.getKey();
  null;
}

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