Proof with simpl; auto; try congruence.
apply prop_ext; split; intro A.
revert s A; induction sigma... intros s [x [y [A [B C]]]].
generalize A; intros [[A0 A1] A2].
destruct a; [exists x; exists y; split; auto | ].
destruct e; destruct e0. inversion B; simpl; auto; try congruence; subst. apply IHsigma.
cut (y = s); [ intro X; rewrite <- X; auto | ].
destruct y as [y1 y2], s as [s1 s2]; f_equal...
exists x; exists y; split...
exists x; exists y; split...
rewrite if_negb; remember (eq_var v v0) as b; destruct b.
2: exists x; exists y; split...
unfold eq_var in Heqb. apply eq_pos_var_eqZ in Heqb. inversion Heqb; subst.
inversion B; subst.
cut (y = s); [ intro X; rewrite <- X; auto | ].
destruct y as [y1 y2], s as [s1 s2]; f_equal... apply IHsigma...
(* <-- *)
revert s A; induction sigma... intros s A. destruct a in A |- *.
destruct A as [x [y [A [B C]]]]. exists x; exists y; split...
remember e as b1; remember  e0 as b2. destruct b1, b2 in A |- *.
rewrite <- lseg_nilnil...
destruct A as [x [y [A [B C]]]]; exists x; exists y; split...
destruct A as [x [y [A [B C]]]]; exists x; exists y; split...
rewrite if_negb in A; remember (eq_var v v0) as b; destruct b.
apply eq_pos_var_eqZ in Heqb; inversion Heqb; subst. rewrite <- lseg_vv...
destruct A as [x [y [A [B C]]]]; exists x; exists y; split...
Qed.

Lemma normalize2_4_sound sc s :
clause_denote sc s -> clause_denote (normalize2_4 sc) s.