public Subscription Add(string name)
        {
            var memento = new SubscriptionMemento
            {
                Id         = Guid.NewGuid(),
                Name       = name,
                CreatedUtc = _getUtcNow(),
                Secret     = Guid.NewGuid().ToString("d")
            };
            var subscription = new Subscription(memento);

            _items.Add(subscription.Id, subscription);
            return(subscription);
        }
 internal Subscription(SubscriptionMemento memento)
 {
     _memento = memento;
 }