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

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

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

static <A, B> Map<A, B> filterKeys(Map<A, B> map, IPred<A> f) {
  ret filterKeys(f, map);
}