Proof using.
intros; induction l1; simpl in *; intuition.
subst_max. specialize (H2 e').
assert (In e' (l1 ++ l2)) by auto with datatypes.
concludes. intuition.
Qed.

Lemma Prefix_In :
forall A (l : list A) l' x,
Prefix l l' ->
In x l ->
In x l'.