public function setAuthnContextDeclRef(string $authnContextDeclRef = null) : void
    {
        if (!empty($this->authnContextDecl)) {
            throw new \Exception(
                'AuthnContextDecl is already registered! May only have either a Decl or a DeclRef, not both!'
            );
        }

        $this->authnContextDeclRef = $authnContextDeclRef;
    }