-"envInitForAlternatives" initialises the environment for a case alternative,
-assuming that the alternative is entered after an evaluation.
-This involves:
-
- - zapping any volatile bindings, which aren't valid.
-
- - zapping the heap usage. It should be restored by a heap check.
-
- - setting the virtual AND real stack pointer fields to the given
- virtual stack offsets. this doesn't represent any {\em code}; it is a
- prediction of where the real stack pointer will be when we come back
- from the case analysis.
-
- - BUT LEAVING the rest of the stack-usage info because it is all
- valid. In particular, we leave the tail stack pointers unchanged,
- becuase the alternative has to de-allocate the original @case@
- expression's stack. \end{itemize}
-