public function listSubscriptions(string $username = null): array
    {
        if (null !== $username) {
            return $this->getApi()->request($this->getApi()->sprintf('/users/:username/subscriptions',
                $this->getActivity()->getOwner(), $this->getActivity()->getRepo(), $username));
        }

        return $this->getApi()->request($this->getApi()
                                             ->sprintf('/user/subscriptions', $this->getActivity()->getOwner(),
                                                 $this->getActivity()->getRepo()));
    }