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