static <A, B> Set<A> keySet(Map<A, B> map) {
  ret map == null ? new HashSet : map.keySet();
}

static Set keySet(O map) {
  ret keys((Map) map);
}

ifclass MultiSet
  static <A> Set<A> keySet(MultiSet<A> ms) {
    ret ms.keySet();
  }
endif

ifclass MultiMap
  static <A, B> Set<A> keySet(MultiMap<A, B> mm) {
    ret mm.keySet();
  }
endif

ifclass SmallestListMultiMap
  static <A, B> Set<A> keySet(SmallestListMultiMap<A, B> mm) {
    ret mm.keySet();
  }
endif

Began life as a copy of #1002042