induction n; intuition; simpl in *.
econstructor.
simpl.
intuition.
econstructor.
eapply app_NoDup; intuition.
eapply map_NoDup; intuition.
eapply H.
apply app_inj_tail in H0.
intuition.
eapply map_NoDup; intuition.
eapply H.
eapply app_inj_tail in H0.
intuition.
apply in_map_iff in H.
apply in_map_iff in H0.
destruct H.
destruct H0.
intuition.
subst.
apply app_inj_tail in H; intuition.
apply in_map_iff in H.
apply in_map_iff in H0.
destruct H.
destruct H0.
intuition.
subst.
apply app_inj_tail in H; intuition.
Qed.

(* Specifications: *)
Lemma ls_last_exists : forall (A : Type)(ls : list A) n,
length ls = (S n) ->
exists a ls', (length ls' = n /\ ls = ls' ++ (a :: nil)).