docs: function(event, extraParams){ // for internal use only
        if( this.length === 0 ){ return; } // empty collections don't need to notify anything
  
        // notify renderer unless removed
        this.cy().notify({
          type: event,
          collection: this.filter(function(){
            return !this.removed();
          })
        });
        
        this.trigger(event, extraParams);
        return this;
      }