docs: function( selector ){
        var eles = this;
        var roots = [];
        for( var i = 0; i < eles.length; i++ ){
          var ele = eles[i];
          if( !ele.isNode() ){
            continue;
          }
  
          var hasEdgesPointingIn = ele.connectedEdges('[target = "' + ele.id() + '"][source != "' + ele.id() + '"]').length > 0;
  
          if( !hasEdgesPointingIn ){
            roots.push( ele );
          }
        }
  
        return new $$.Collection( this._private.cy, roots ).filter( selector );
      }