Check if a subscription exists
subscriptions_exists(subscription)logical TRUE if the subscription exist
character, Subscription Required, subscription name or instance of
a Subscription object
Other Subscription functions:
subscriptions_ack(),
subscriptions_create(),
subscriptions_delete(),
subscriptions_detach(),
subscriptions_get(),
subscriptions_list(),
subscriptions_modify_ack_deadline(),
subscriptions_modify_pushconf(),
subscriptions_patch(),
subscriptions_pull(),
subscriptions_seek()