+lemma prove_in_segment:
+ ∀O:half_ordered_set.∀s:segment O.∀x:O.
+ (seg_l O s) ≤≤ x → x ≤≤ (seg_u O s) → x ∈ s.
+intros; unfold; cases (wloss_prop O); rewrite < H2;
+split; assumption;
+qed.
+
+lemma cases_in_segment:
+ ∀C:half_ordered_set.∀s:segment C.∀x. x ∈ s → (seg_l C s) ≤≤ x ∧ x ≤≤ (seg_u C s).
+intros; unfold in H; cases (wloss_prop C) (W W); rewrite<W in H; [cases H; split;assumption]
+cases H; split; assumption;
+qed.
+