-- newtype N a = MkN (Tree [a]) deriving( Foo Int )
--
-- The newtype gives an FC axiom looking like
--- axiom CoN a :: N a :=: Tree [a]
+-- axiom CoN a :: N a ~ Tree [a]
-- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
--
-- So all need is to generate a binding looking like:
-- make_coercion
-- The inst_head looks like (C s1 .. sm (T a1 .. ak))
-- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
- -- with kind (C s1 .. sm (T a1 .. ak) :=: C s1 .. sm <rep_ty>)
+ -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
-- where rep_ty is the (eta-reduced) type rep of T
-- So we just replace T with CoT, and insert a 'sym'
-- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced