induction l.
+ intros l' l1 l2 Hyp; simpl in Hyp; inversion_clear Hyp; auto.
+ intros l' l1 l2 Hyp.
simpl in Hyp.
destruct l' as [ | y ys] eqn:L'.
- inversion Hyp.
- destruct l1 eqn:L1.
* destruct (lengthSplit l ys); simpl in *.
+ destruct p; congruence.
+ congruence.
* pose proof IHl ys l0 l2; clear IHl.
destruct (lengthSplit l ys) eqn:LenSplit; simpl in *.
+ inversion Hyp. destruct p. inversion H1. subst.
rewrite H; auto.
+ inversion Hyp.
Qed.

Lemma lengthSplit3 : forall {A : Type} (l l' l1 l2 : list A),
lengthSplit l l' = Some (l1, l2) -> length l1 = length l.