pub fn transitivity<A: Prop, B: Prop, C: Prop>( Sq: Sq<A, B>, Sq: Sq<B, C> ) -> Sq<A, C>
(a ¬> b) ⋀ (b ¬> c) => (a ¬> c).
(a ¬> b) ⋀ (b ¬> c) => (a ¬> c)