Allow cycles in class declarations
In particular, I would like to have a class hierarchy that looks like:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
type family Scalar x
class Field (Scalar x) => Vector x
class Vector x => Field xIt is the case that every field is a vector, and the scalar associated with every vector is a field. But we can't currently enforce that.