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

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

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

Began life as a copy of #1002042