code: function( fn ){
        var _p = this._private;
        var pass = _p.pass[0];
  
        return this.map( fn ).then(function( include ){
          var ret = [];
  
          for( var i = 0; i < pass.length; i++ ){
            var datum = pass[i];
            var incDatum = include[i];
  
            if( incDatum ){
              ret.push( datum );
            }
          }
  
          return ret;
        });
      }
docs: parallel version of array.filter()