static <A, B> A firstKeyWithValue(Map<A, B> map, B value) {
  for (A key : keys(map))
    if (eq(map.get(key), value))
      ret key;
  null;
}

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