+Note [Fusing case continuations]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's important to fuse two successive case continuations when the
+first has one alternative. That's why we call prepareCaseCont here.
+Consider this, which arises from thunk splitting (see Note [Thunk
+splitting] in WorkWrap):
+
+ let
+ x* = case (case v of {pn -> rn}) of
+ I# a -> I# a
+ in body
+
+The simplifier will find
+ (Var v) with continuation
+ Select (pn -> rn) (
+ Select [I# a -> I# a] (
+ StrictBind body Stop
+
+So we'll call mkDupableCont on
+ Select [I# a -> I# a] (StrictBind body Stop)
+There is just one alternative in the first Select, so we want to
+simplify the rhs (I# a) with continuation (StricgtBind body Stop)
+Supposing that body is big, we end up with
+ let $j a = <let x = I# a in body>
+ in case v of { pn -> case rn of
+ I# a -> $j a }
+This is just what we want because the rn produces a box that
+the case rn cancels with.
+
+See Trac #4957 a fuller example.
+