public function addLineNumbers()
    {
        $lengthToPad = strlen(count($this->lines));
        $c = 0;
        foreach( $this->lines as &$line ) {
            $c++;
            $line = sprintf(
                "%s %s",
                str_pad( (string) $c, $lengthToPad, ' ', STR_PAD_LEFT ),
                $line
            );
        }
        return $this;
    }