public function toHTML($separator = '<br/>') {
        $output = '';

        foreach ($this->diff as $entry) {
            $element = '';
            if ($entry instanceof UnmodifiedEntry) {
                $element = 'span';
            } elseif ($entry instanceof DeletedEntry) {
                $element = 'del';
            } elseif ($entry instanceof InsertedEntry) {
                $element = 'ins';
            }
            $output .= '<' . $element . '>' . $entry . '</' . $element . '>' . $separator;
        }

        return $output;
    }