static <A, B> NavigableSet<A> navigableKeys(Map<A, B> map) {
  ret map == null ? new TreeSet : map.navigableKeySet();
}

static NavigableSet navigableKeys(O map) {
  ret navigableKeys((Map) map);
}

ifclass MultiSet
  static <A> NavigableSet<A> navigableKeys(MultiSet<A> ms) {
    ret ms.navigableKeySet();
  }
endif

ifclass MultiMap
  static <A, B> NavigableSet<A> navigableKeys(MultiMap<A, B> mm) {
    ret mm.navigableKeySet();
  }
endif