pub fn transitivity<A: Prop, B: Prop, C: Prop>( (f0, f1): Eq<A, B>, (g0, g1): Eq<B, C>) -> Eq<A, C>
(a = b) ∧ (b = c) => (a = c).
(a = b) ∧ (b = c) => (a = c)