On concurrency tokens

Microsoft states for .NET Framework 4: "WCF Data Services supports the optimistic concurrency model by enabling you to define a concurrency token for an entity. This concurrency token, which includes one or more properties of the entity, is used by the data service to determine whether a change has occurred in the data that is being requested, updated, or deleted."
Now this is interesting as it suggests a growing realisation that more needs to be done about optimistic concurrency. But alas, all that is done here is to declare a property as useful for applications to test: it comes with no guarantees.
Pyrrho already has (mandatory) optimistic concurrency control, but still, it is an attractive notion to have a test to perform during a transaction ("is it already too late to commit?").
So from the next version (4.5) of Pyrrho, there will be a boolean property of transactions. Requesting the value of Transaction.Conflict will cause a server round trip to discover if other transaction commits have already made committing this transaction impossible.
Whatever the result of this test, the Transaction will remain open as it probably still contains useful information, until Commit or Rollback is called.