+Note [Superclasses of derived instance]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In general, a derived instance decl needs the superclasses of the derived
+class too. So if we have
+ data T a = ...deriving( Ord )
+then the initial context for Ord (T a) should include Eq (T a). Often this is
+redundant; we'll also generate an Ord constraint for each constructor argument,
+and that will probably generate enough constraints to make the Eq (T a) constraint
+be satisfied too. But not always; consider:
+
+ data S a = S
+ instance Eq (S a)
+ instance Ord (S a)
+
+ data T a = MkT (S a) deriving( Ord )
+ instance Num a => Eq (T a)
+
+The derived instance for (Ord (T a)) must have a (Num a) constraint!
+Similarly consider:
+ data T a = MkT deriving( Data, Typeable )
+Here there *is* no argument field, but we must nevertheless generate
+a context for the Data instances:
+ instance Typable a => Data (T a) where ...
+