instance (Additive a, Additive b, Additive c, Additive d) => Additive (a, b, c, d) where
zero = (zero, zero, zero, zero)
(a0, b0, c0, d0) + (a1, b1, c1, d1) = (a0+a1, b0+b1, c0+c1, d0+d1)
instance (Additive a, Additive b, Additive c, Additive d) => Additive (a, b, c, d) where
zero = (zero, zero, zero, zero)
(a0, b0, c0, d0) + (a1, b1, c1, d1) = (a0+a1, b0+b1, c0+c1, d0+d1)