proposition: Lemma ordinal_segment E x: ordinalp E -> inc x E -> segment (ordinal_o E) x = x.
proof: Proof. move=> ox xE;rewrite ordinal_same_wo //. by apply: ordinal_segment1 => //; apply: ordinal_trans_dec. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)