public function verify(array $credentials, Closure $callback)
    {
        $user = $this->validateVerification($credentials);

        if (!$user instanceof Contracts\RequiresEmailVerification) {
            return $user;
        }

        call_user_func($callback, $user);

        $this->tokens->delete($credentials['token']);

        return Contracts\VerificationBroker::VERIFIED;
    }