docs: public Futures subRef( Vec vec, Futures fs ) {
      assert fs != null : "Future should not be null!";
      if ( vec.masterVec() != null ) subRef(vec.masterVec(), fs);
      int cnt = _refcnt.get(vec)._val-1;
      if ( cnt > 0 ) {
        _refcnt.put(vec,new IcedInt(cnt));
      } else {
        UKV.remove(vec._key,fs);
        _refcnt.remove(vec);
      }
      return fs;
    }