+ --
+ -- BUT WATCH OUT! If the method type mentions the class variable, then
+ -- this optimisation is not right. Consider
+ -- class C a where
+ -- op :: Eq a => a
+ --
+ -- instance C Int where
+ -- op = op
+ -- The occurrence of 'op' on the rhs gives rise to a constraint
+ -- op at Int
+ -- The trouble is that the 'meth_inst' for op, which is 'available', also
+ -- looks like 'op at Int'. But they are not the same.