intros. rewrite lseg_unroll.
apply pred_ext. apply orp_left; auto.
rewrite andp_assoc.
do 2 (apply derives_extract_prop; intro).
congruence.
apply orp_right2. auto.
Qed.

Lemma semax_lseg_nonnull (ls: listspec list_structid list_link list_token):
forall (Espec: OracleKind)
Delta P Q dsh psh s v R c Post,
ENTAIL Delta, PROPx P (LOCALx Q
(SEPx (lseg ls dsh psh s v nullval :: R))) |--
!!(typed_true (tptr list_struct) v)  ->
(forall (h: elemtype ls) (r: list (val * elemtype ls)) (y: val),
s=(v,h)::r -> is_pointer_or_null y ->
semax Delta
(PROPx P (LOCALx Q
(SEPx (list_token dsh v :: list_cell ls dsh h v ::
field_at psh list_struct (StructField list_link :: nil) (valinject (nested_field_type list_struct (StructField list_link :: nil)) y) v ::
lseg ls dsh psh r y nullval ::
R)))) c Post) ->
semax Delta
(PROPx P (LOCALx Q (SEPx (lseg ls dsh psh s v nullval :: R))))
c Post.