[Zope-dev] BTrees and ConflictErrors.

I thought the newer BTrees code was meant to deal with conflicts from two entries being added at the same time? I'm using CoreSessionTracking 0.8 with an external data source on a bsddb3Storage.Minimal storage mounted via ZEO (using the current Zope2.3 code). If two sessions are created around the same time, I'm seeing ConflictErrors - there doesn't seem to be any attempt to invoke the resolveConflict code. Any ideas what I'm missing here? Anthony