CorePrep: `cpeRhsE Case{}` uses cpeBody for scrutinee, but Note [CorePrep invariants] suggests cpeApp

This is the grammar in Note [CorePrep invariants]:

Here is the syntax of the Core produced by CorePrep:

    Trivial expressions
       arg ::= lit |  var
              | arg ty  |  /\a. arg
              | truv co  |  /\c. arg  |  arg |> co

    Applications
       app ::= lit  |  var  |  app arg  |  app ty  | app co | app |> co

    Expressions
       body ::= app
              | let(rec) x = rhs in body     -- Boxed only
              | case app of pat -> body
              | /\a. body | /\c. body
              | body |> co

    Right hand sides (only place where value lambdas can occur)
       rhs ::= /\a.rhs  |  \x.rhs  |  body

(Side note: What is truv in truv co? I think it should be app co.)

This is the main case of cpeRhsE Case{}, the case that prep's Case expressions in GHC.CoreToStg.Prep:

cpeRhsE env (Case scrut bndr ty alts)
  = do { (floats, scrut') <- cpeBody env scrut
       ... Case scrut' ...

But it uses cpeBody, suggesting that a prep'd Case accepts a body non-terminal. Which is it? At the least I'd expect an assertion.

But realistically, I think our pipeline can in fact cope with entire bodys in the scrutinee, not just app (although that would feel cleaner to me).