code: function _addRef(refer, refee) {
          var item = _refMap[refee._id];
          if (!item) {
              item = {};
              _refMap[refee._id] = item;
          }
          var count = item[refer._id];
          if (count) {
              item[refer._id] = count + 1;
          } else {
              item[refer._id] = 1;
          }
      }
docs: Add a reference to the inverted index. (increase refCount by one)
  
  @private
  @param {Core.Element} refer Referencing element.
  @param {Core.Element} refee Referenced element.