proposition: Lemma ordinal_sub x y: ordinalp x -> ordinalp y -> sub x y -> x = y \/ inc x y.
proof: Proof. move=> ox oy xy; alt_tac nxy. exact: (oy _ xy (ordinal_transitive ox) nxy). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)