private void Unsubscribe(OperationStoreSession session)
 {
     lock (_sync)
     {
         _sessions = _sessions.Remove(session);
     }
 }
            public IDisposable Subscribe(IObserver <OperationUpdate> observer)
            {
                var session = new OperationStoreSession(observer, Unsubscribe);

                lock (_sync)
                {
                    _sessions = _sessions.Add(session);
                }

                return(session);
            }