private function parseSubject(DOMElement $xml) : void
    {
        /** @var \DOMElement[] $subject */
        $subject = Utils::xpQuery($xml, './saml_assertion:Subject');
        if (empty($subject)) {
            /* No Subject node. */

            return;
        } elseif (count($subject) > 1) {
            throw new \Exception('More than one <saml:Subject> in <saml:Assertion>.');
        }
        $subject = $subject[0];

        /** @var \DOMElement[] $nameId */
        $nameId = Utils::xpQuery(
            $subject,
            './saml_assertion:NameID | ./saml_assertion:EncryptedID/xenc:EncryptedData'
        );
        if (count($nameId) > 1) {
            throw new \Exception('More than one <saml:NameID> or <saml:EncryptedID> in <saml:Subject>.');
        } elseif (!empty($nameId)) {
            $nameId = $nameId[0];
            if ($nameId->localName === 'EncryptedData') {
                /* The NameID element is encrypted. */
                $this->encryptedNameId = $nameId;
            } else {
                $this->nameId = new NameID($nameId);
            }
        }

        /** @var \DOMElement[] $subjectConfirmation */
        $subjectConfirmation = Utils::xpQuery($subject, './saml_assertion:SubjectConfirmation');
        if (empty($subjectConfirmation) && empty($nameId)) {
            throw new \Exception('Missing <saml:SubjectConfirmation> in <saml:Subject>.');
        }

        foreach ($subjectConfirmation as $sc) {
            $this->SubjectConfirmation[] = new SubjectConfirmation($sc);
        }
    }