public void When()
        {
            // given
            var tracker = new PositionTracker(new StreamPositionTagger("stream1"));

            tracker.Update(
                new ProjectionMessage.Projections.CommittedEventReceived(
                    Guid.NewGuid(), new EventPosition(100, 50), "stream1", 1, false,
                    new Event(Guid.NewGuid(), "eventtype", false, new byte[0], new byte[0])));
            _tag             = tracker.LastTag;
            _tagger          = new StreamPositionTagger("stream1");
            _positionTracker = new PositionTracker(_tagger);
            // when

            _positionTracker.UpdateByCheckpointTag(_tag);
        }