Completely positive projections and biproducts

The recently introduced CP*–construction unites quantum channels and classical systems, subsuming the earlier CPM–construction in categorical quantum mechanics. We compare this construction to two earlier attempts at solving this problem: freely adding biproducts to CPM, and freely splitting idempotents in CPM. The CP*–construction embeds the former, and embeds into the latter, but neither embedding is an equivalence in general.