I can't get Contravariant function args

In C++ I'm having a hell of time getting these new contravariant function parameters working.

Yeah, imagine a complex state with 20 MB of seemingly random data. You need to "move forwards", producing the next state. To do this, you have to run a series of functions that iterate in various ways through portions of the state and decide what to change. If you can afford side effects, and the algorithm allows it, you change your state in place. If you can't have side effects, you have to produce a new 20 MB of state, then trash the old one.