public function toXML(DOMElement $parent) : ?DOMElement
    {
        if (!empty($this->IPHint)
         || !empty($this->DomainHint)
         || !empty($this->GeolocationHint)
         || !empty($this->children)
        ) {
            $doc = $parent->ownerDocument;

            $e = $doc->createElementNS(Common::NS, 'mdui:DiscoHints');
            $parent->appendChild($e);

            foreach ($this->getChildren() as $child) {
                $child->toXML($e);
            }

            Utils::addStrings($e, Common::NS, 'mdui:IPHint', false, $this->IPHint);
            Utils::addStrings($e, Common::NS, 'mdui:DomainHint', false, $this->DomainHint);
            Utils::addStrings($e, Common::NS, 'mdui:GeolocationHint', false, $this->GeolocationHint);

            return $e;
        }

        return null;
    }