proposition: Lemma ap_pr3 a b c y (x := [a,b,c]): y < x -> y + x = x.
proof: Proof. by case: y => // a' b' c' n' d' /=; rewrite /x psi_lt1 => ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)