public function onTrial(string $subscription = 'default', ?string $plan = null): bool
    {
        if (func_num_args() === 0 && $this->onGenericTrial()) {
            return true;
        }
        $subscription = $this->subscription($subscription);
        if (is_null($plan)) {
            return $subscription && $subscription->onTrial();
        }

        return $subscription && $subscription->onTrial() &&
            $subscription->stripePlan === $plan;
    }