+Note [WildCard binders]
+~~~~~~~~~~~~~~~~~~~~~~~
+The program to be simplified may have wild binders
+ case e of wild { p -> ... }
+We want to *rename* them away, so that there are no
+occurrences of 'wild-id' (with wildCardKey). The easy
+way to do that is to start of with a representative
+Id in the in-scope set
+
+There can be be *occurrences* of wild-id. For example,
+MkCore.mkCoreApp transforms
+ e (a /# b) --> case (a /# b) of wild { DEFAULT -> e wild }
+This is ok provided 'wild' isn't free in 'e', and that's the delicate
+thing. Generally, you want to run the simplifier to get rid of the
+wild-ids before doing much else.
+
+It's a very dark corner of GHC. Maybe it should be cleaned up.
+
+\begin{code}