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

static <A, B> Map<A, B> filterMapByFunctionOnKey(O f, Map<A, B> map) {
  ret filterMapByFunctionOnKey(map, f);
}