proposition: Lemma ST1_leP x y: gle ST1_order x y <-> ST1_le x y.
proof: Proof. split; first by move/graph_on_P1 => []. by move => h; move:(h) =>[ha hb _]; apply/graph_on_P1. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)