Proof using.
induction l1; intros; simpl in *; intuition; eauto.
subst. find_insterU. conclude_using ltac:(apply in_app_iff; intuition eauto).
intuition.
Qed.

Lemma sorted_app_sorted_app_in1_in2 :
forall l1 l2 l3 e e',
sorted (l1 ++ l3) ->
sorted (l2 ++ l3) ->
In e l1 ->
In e' (l2 ++ l3) ->
eIndex e' = eIndex e ->
In e' l2.