-- Recursive case, (n+2) x (n+2) matrices.
  instance (Eq a, Ring.C a, Arity n)
         => Determined (Mat (S (S n)) (S (S n))) a where
  determinant m =
    ...
    -- Recursive algorithm, the i,jth minor has dimension
    -- (n+1) x (n+1).
    foo bar (determinant (minor m i j))

I get an error stating that I'm missing an instance:

  Could not deduce (Determined (Mat (S n) (S n)) a)
  ...