What should Process::LongitudinalProfile count?

Currently this process counts "Tracks" it receives from Cascade::Step. So essentially it counts Steps. But this is an artifical non-physical quantity. If we change the number of Steps for technical reasons, we would scale the LongitudinalProfile. This can't be the desired effect. LongitudinalProfile has to count something physical. The obvious answer is "particles per dX", but in the context of the Cascade, does this mean "newly produced particles per dX"? I think so.