public function last($cb=null)
    {
        if ($cb==null) {
            return $this->isNotEmpty()?$this->rows[$this->count()-1]:null;
        } else {
            //method returns the last element in the collection that passes a given truth test:
            $count = $this->count();
            for ($i=0;$i<$count;$i++) {
                if ($cb($this->rows[$count-$i], $count-$i)===true) {
                    return $this->rows[$count-$i];
                }
            }
        }
        return null;
    }