pub fn eq_right<A: Prop, B: Prop, C: Prop>( (ab, ba): Eq<A, B> ) -> Eq<And<C, A>, And<C, B>>
(a == b) => (c ∧ a) == (c ∧ b).
(a == b) => (c ∧ a) == (c ∧ b)