+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.