code: function unmergeOne(ele) {
      ele = ele[0];
      var _p = this._private;
      var id = ele._private.data.id;
      var map = _p.map;
      var entry = map.get(id);
  
      if (!entry) {
        return this; // no need to remove
      }
  
      var i = entry.index;
      this.unmergeAt(i);
      return this;
    }
docs: remove single ele in place in calling collection