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 lseg_cons_eq (ls: listspec list_structid list_link list_token):
forall dsh psh h r x z ,
lseg ls dsh psh (h::r) x z =
!!(x = fst h /\ ~ ptr_eq x z) &&
(EX  y : val,
!!(is_pointer_or_null y) &&
list_token dsh x * list_cell ls dsh (snd h) x * field_at psh list_struct (StructField list_link :: nil) (valinject (nested_field_type list_struct (StructField list_link :: nil)) y) x *
lseg ls dsh psh r y z).