+Note [Subtle interaction of recursion and overlap]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this
+ class C a where { op1,op2 :: a -> a }
+ instance C a => C [a] where
+ op1 x = op2 x ++ op2 x
+ op2 x = ...
+ intance C [Int] where
+ ...
+
+When type-checking the C [a] instance, we need a C [a] dictionary (for
+the call of op2). If we look up in the instance environment, we find
+an overlap. And in *general* the right thing is to complain (see Note
+[Overlapping instances] in InstEnv). But in *this* case it's wrong to
+complain, because we just want to delegate to the op2 of this same
+instance.
+
+Why is this justified? Because we generate a (C [a]) constraint in
+a context in which 'a' cannot be instantiated to anything that matches
+other overlapping instances, or else we would not be excecuting this
+version of op1 in the first place.
+
+It might even be a bit disguised:
+
+ nullFail :: C [a] => [a] -> [a]
+ nullFail x = op2 x ++ op2 x
+
+ instance C a => C [a] where
+ op1 x = nullFail x
+
+Precisely this is used in package 'regex-base', module Context.hs.
+See the overlapping instances for RegexContext, and the fact that they
+call 'nullFail' just like the example above. The DoCon package also
+does the same thing; it shows up in module Fraction.hs
+
+Conclusion: when typechecking the methods in a C [a] instance, we want
+to have C [a] available. That is why we have the strange local
+definition for 'this' in the definition of op1_i in the example above.
+We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
+we supply 'this' as a given dictionary. Only needed, though, if there
+are some type variales involved; otherwise there can be no overlap and
+none of this arises.
+