cet-is » art

This is a heads up for something that will be important later on.

Mu2e currently has one Run scope data product that holds a summary of the Geant4 geometry; it is an std::vector<mu2e::PhysicalVolumeInfo>. A number of our event scope data products currently hold indices into this Run data product. Sometime soon we expect to have art::Ptr from an event data product into run data products. At that time we need to deal with event mixing and these objects.

First, I would like it to be an error if people try to mix files in which this particular data product differs. So we only need one copy, not multiple copies, of the run data product. We might also need a remapper for this sort of Ptr ( or maybe not, if, in all files, the run data product has the same product ID)?