public function getAlternateUriSet()
    {
        $uris = [];
        if (isset($this->principalProperties['{DAV:}alternate-URI-set'])) {
            $uris = $this->principalProperties['{DAV:}alternate-URI-set'];
        }

        if (isset($this->principalProperties['{http://sabredav.org/ns}email-address'])) {
            $uris[] = 'mailto:'.$this->principalProperties['{http://sabredav.org/ns}email-address'];
        }

        return array_unique($uris);
    }