docs: function(dataset){
      this.sources.push(dataset);
      // add event listeners to source
      if (this.listen.source)
        dataset.addHandler(this.listen.source, this);
  
      // process new dataset objects and update member map
      var memberMap = this.members_;
      for (var objectId in dataset.items_)
      {
        // check: is this object already known
        if (memberMap[objectId])
        {
          // item exists -> increase dataset links count
          memberMap[objectId].count++;
        }
        else
        {
          // add to source map
          memberMap[objectId] = {
            count: 1,
            isMember: false,
            object: dataset.items_[objectId]
          };
        }
      }
  
      return true;
    }