[Haskell-cafe] Reverse unification question

[Martijn van Steenbergen]

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.