+ inversion H. subst l2. inversion H0.
- destruct l0; simpl in H5; inversion H5.
- clear H6. destruct l0; inversion H5.
* subst. simpl in *. inversion H2. subst; inversion H4.
constructor.
constructor.
subst l3; auto.
-  auto.
revert l3.
induction H; intros l3 H0.
+ admit.
+ inversion H0.
- constructor.
apply IHsubseq; auto.
- constructor. apply
Inductive foo: Type :=
bar: foo -> foo.

Lemma blah: forall x, x=O -> ~ exists y, x = S y .
intros x H H0; destruct x.