First, we assume that syntactic realization serves only to distinguish the Cp from all other referents, which are ranked on the same level: Thus effectively subject > others.
secondly, we assume that the system already knows, from the argument structure of the proposition, which entities can occur in subject position: in realizing a proposition ban(fda, elixir), both arguments are potential Cps because active and passive realizations are both allowed; for contain(elixir, gestodene), only elixir is a potential Cp because we disallow Gestodene is contained by Elixir.