+Note [Worker-wrapper for bottoming functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We used not to split if the result is bottom.
+[Justification: there's no efficiency to be gained.]
+
+But it's sometimes bad not to make a wrapper. Consider
+ fw = \x# -> let x = I# x# in case e of
+ p1 -> error_fn x
+ p2 -> error_fn x
+ p3 -> the real stuff
+The re-boxing code won't go away unless error_fn gets a wrapper too.
+[We don't do reboxing now, but in general it's better to pass an
+unboxed thing to f, and have it reboxed in the error cases....]