public function move ( $dx, $dy ) {
        if ( $dx < 0 ) {
            $this->left(abs($dx));
        } else {
            $this->right(abs($dx));
        }
        if ( $dy < 0 ) {
            $this->up(abs($dy));
        } else {
            $this->down(abs($dy));
        }
        return $this;
    }