private async Task WriteCheckpoint(Position?checkpoint)
 {
     await AppendToStream("checkpoints",
                          ExpectedVersion.Any,
                          new CheckpointReached
     {
         CheckpointToken = checkpoint.ToCheckpointToken(),
         CorrelationId   = _correlationId,
         ProcessId       =
             ProcessHandler <OrderFulfillment, CompareablePosition> .DefaultBuildProcessManagerId(
                 _correlationId)
     });
 }