Interface ScreenAnswerAcrossPush<Args, A, E, R>

A request whose answer crosses a push: one screen's enter sends it and returns while it is in flight, and the screen that screen pushes next takes its answer instead of sending the request itself (sendAcrossPush and takeAcrossPush, members of every screen's enter arguments). The declaration is the one fact both screens share: the request, and its identity — a screen receives only an answer to the declaration it takes.