static <A, B> Map<A, B> filterMap(Map<A, B> map, O f) {
  Map<A, B> m2 = similarEmptyMap(map);
  for (A a : keys(map)) {
    B b = map.get(a);
    if (isTrue(callF(f, a)))
      m2.put(a, b);
  }
  ret m2;
}