private function appendNode(\DOMNode $parentNode, $data, string $nodeName, string $key = null): bool
    {
        $node = $this->dom->createElement($nodeName);
        if (null !== $key) {
            $node->setAttribute('key', $key);
        }
        $appendNode = $this->selectNodeType($node, $data);
        // we may have decided not to append this node, either in error or if its $nodeName is not valid
        if ($appendNode) {
            $parentNode->appendChild($node);
        }

        return $appendNode;
    }