Section before.
Variable A : Type.
Proof using.
induction l; intros; simpl in *; intuition.
Qed.
Proof using.
induction l; intros; simpl in *; intuition; subst; try congruence.
- exists nil. simpl. find_apply_lem_hyp in_split. break_exists. subst. eauto.
- exists nil. simpl. find_apply_lem_hyp in_split. break_exists. subst. eauto.
- exists nil. simpl. find_apply_lem_hyp in_split. break_exists. subst. eauto.
- eapply_prop_hyp In In; eauto. break_exists. subst.
exists (a :: x0), x1, x2. auto.
Qed.
Proof using.
induction xs; intros; simpl in *; intuition.
Qed.

Lemma before_2_3_insert :
forall xs ys zs x y a b,
before(A:=A) a b (xs ++ ys ++ zs) ->
b <> x ->
b <> y ->
before a b (xs ++ x :: ys ++ y :: zs).