public function addDocumentation($input_node, $documentation)
    {
        if ($input_node === $this) {
            $node = $this->_dom->documentElement;
        } else {
            $node = $input_node;
        }

        $doc = $this->_dom->createElement('documentation');
        $doc_cdata = $this->_dom->createTextNode($documentation);
        $doc->appendChild($doc_cdata);

        if ($node->hasChildNodes()) {
            $node->insertBefore($doc, $node->firstChild);
        } else {
            $node->appendChild($doc);
        }

        return $doc;
    }