public function getResponseType(string $responseType): GrantInterface
    {
        if ($this->hasResponseType($responseType)) {
            return $this->responseTypes[$responseType];
        }

        // If we reach here... then no grant was found. Not good!
        throw OAuth2Exception::unsupportedResponseType(sprintf(
            'Response type "%s" is not supported by this server',
            $responseType
        ));
    }