pub fn seshatic<A: Prop, B: Prop>(sq: Sq<A, B>) -> Seshatic<A, B>
(a ¬> b) => (¬(a ~~ a) ⋁ ¬(b ~~ b)).
(a ¬> b) => (¬(a ~~ a) ⋁ ¬(b ~~ b))