FastInt -- Size to subtract if result is scrutinised
-- by a case expression
+-- subtract the discount before deciding whether to bale out. eg. we
+-- want to inline a large constructor application into a selector:
+-- tup = (a_1, ..., a_99)
+-- x = case tup of ...
+--
mkSizeIs max n xs d | (n -# d) ># max = TooBig
| otherwise = SizeIs n xs d