public override CheckpointTag MakeCheckpointTag(
     CheckpointTag previous, ReaderSubscriptionMessage.CommittedEventDistributed committedEvent)
 {
     if (!_streams.Contains(committedEvent.Data.PositionStreamId))
         throw new InvalidOperationException(
             string.Format("Invalid stream '{0}'", committedEvent.Data.EventStreamId));
     return previous.UpdateStreamPosition(
         committedEvent.Data.PositionStreamId, committedEvent.Data.PositionSequenceNumber);
 }
        public void updated_position_is_correct()
        {
            var updated = _a1b1.UpdateStreamPosition("a", 2);

            Assert.AreEqual(2, updated.Streams["a"]);
        }