public function previousXY()
    {
        if (($this->keyY()-1)>0) {
            $this->seekY($this->keyY()-1);
        } else {
            if (($this->keyX()-1)>0) {
                $this->rewindY();
                $this->seekX($this->keyX()-1);
            } else {
                $this->x = null;
                $this->y = null;
            }
        }
        return $this;
    }