static Map mapLike mapKeys(O func, Map map) {
  Map m = similarEmptyMap(map);
  for (O key : keys(map))
    m.put(callF(func, key), map.get(key));
  ret m;
}

static Map mapKeys(Map map, O func) {
  ret mapKeys(func, map);
}

static <A, B, C> Map<B, C> mapKeys(Map<A, C> map, IF1<A, B> func) {
  ret mapKeys(map, (O) func);
}