+Note [Overlap and deriving]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider some overlapping instances:
+ data Show a => Show [a] where ..
+ data Show [Char] where ...
+
+Now a data type with deriving:
+ data T a = MkT [a] deriving( Show )
+
+We want to get the derived instance
+ instance Show [a] => Show (T a) where...
+and NOT
+ instance Show a => Show (T a) where...
+so that the (Show (T Char)) instance does the Right Thing
+
+It's very like the situation when we're inferring the type
+of a function
+ f x = show [x]
+and we want to infer
+ f :: Show [a] => a -> String
+
+BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
+ the context for the derived instance.
+ Hence tcInstSkolTyVars not tcInstSuperSkolTyVars
+