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

// convenience shortcut for keys_gen
static Set keys(O map) {
  ret keys((Map) map);
}

ifclass MultiSet
  static <A> Set<A> keys(MultiSet<A> ms) {
    ret ms == null ?: ms.keySet();
  }
endif

ifclass IMultiMap
  static <A, B> Set<A> keys(IMultiMap<A, B> mm) {
    ret mm == null ?: mm.keySet();
  }
endif

ifclass SmallestListMultiMap
  static <A, B> Set<A> keys(SmallestListMultiMap<A, B> mm) {
    ret mm == null ?: mm.keySet();
  }
endif