+There is a danger though. Consider
+ let v = x +# y
+ in case (f v) of w -> ...v...v...
+And suppose that (f v) expands to just v. Then we'd like to
+use 'w' instead of 'v' in the alternative. But it may be too
+late; we may have substituted the (cheap) x+#y for v in the
+same simplifier pass that reduced (f v) to v.
+
+I think this is just too bad. CSE will recover some of it.
+
+Note [Binder swap on GlobalId scrutinees]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When the scrutinee is a GlobalId we must take care in two ways
+
+ i) In order to *know* whether 'x' occurs free in the RHS, we need its
+ occurrence info. BUT, we don't gather occurrence info for
+ GlobalIds. That's what the (small) occ_scrut_ids set in OccEnv is
+ for: it says "gather occurrence info for these.
+
+ ii) We must call localiseId on 'x' first, in case it's a GlobalId, or
+ has an External Name. See, for example, SimplEnv Note [Global Ids in
+ the substitution].
+
+Historical note [no-case-of-case]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We *used* to suppress the binder-swap in case expressoins when
+-fno-case-of-case is on. Old remarks:
+ "This happens in the first simplifier pass,
+ and enhances full laziness. Here's the bad case:
+ f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
+ If we eliminate the inner case, we trap it inside the I# v -> arm,
+ which might prevent some full laziness happening. I've seen this
+ in action in spectral/cichelli/Prog.hs:
+ [(m,n) | m <- [1..max], n <- [1..max]]
+ Hence the check for NoCaseOfCase."
+However, now the full-laziness pass itself reverses the binder-swap, so this
+check is no longer necessary.
+
+Historical note [Suppressing the case binder-swap]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This old note describes a problem that is also fixed by doing the
+binder-swap in OccAnal:
+
+ There is another situation when it might make sense to suppress the
+ case-expression binde-swap. If we have
+
+ case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
+ ...other cases .... }
+
+ We'll perform the binder-swap for the outer case, giving
+
+ case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
+ ...other cases .... }
+
+ But there is no point in doing it for the inner case, because w1 can't
+ be inlined anyway. Furthermore, doing the case-swapping involves
+ zapping w2's occurrence info (see paragraphs that follow), and that
+ forces us to bind w2 when doing case merging. So we get
+
+ case x of w1 { A -> let w2 = w1 in e1
+ B -> let w2 = w1 in e2
+ ...other cases .... }
+
+ This is plain silly in the common case where w2 is dead.
+
+ Even so, I can't see a good way to implement this idea. I tried
+ not doing the binder-swap if the scrutinee was already evaluated
+ but that failed big-time:
+
+ data T = MkT !Int
+
+ case v of w { MkT x ->
+ case x of x1 { I# y1 ->
+ case x of x2 { I# y2 -> ...
+
+ Notice that because MkT is strict, x is marked "evaluated". But to
+ eliminate the last case, we must either make sure that x (as well as
+ x1) has unfolding MkT y1. THe straightforward thing to do is to do
+ the binder-swap. So this whole note is a no-op.
+
+It's fixed by doing the binder-swap in OccAnal because we can do the
+binder-swap unconditionally and still get occurrence analysis
+information right.
+
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