[Haskell-cafe] Reverse unification question

Dear café, Given:

instance Category C y :: forall r. C r (A -> r)

I am looking for the types of x and z such that:

x . y :: forall r. C r r y . z :: forall r. C r r

Can you help me find such types? I suspect only one of them exists. Less importantly, at least to me at this moment: how do I solve problems like these in general? Thank you, Martijn.