Definition lseg_cons (ls: listspec list_structid list_link list_token) sh (l: list (elemtype ls)) (x z: val) : mpred :=
!! (~ ptr_eq x z) &&
EX h:(elemtype ls), EX r:list (elemtype ls), EX y:val,
!!(l=h::r  /\ is_pointer_or_null y) &&
list_token sh x * list_cell ls sh h x *
field_at sh list_struct (StructField list_link :: nil) (valinject (nested_field_type list_struct (StructField list_link :: nil)) y) x *
lseg ls sh r y z.

(* Specifications: *)
Lemma lseg_unroll (ls: listspec list_structid list_link list_token): forall sh l x z ,
lseg ls sh l x z =
(!! (ptr_eq x z) && !! (l=nil) && emp) || lseg_cons ls sh l x z.
Lemma lseg_unroll_nonempty1 (ls: listspec list_structid list_link list_token):
forall p P sh h (tail: list (elemtype ls)) v1 v2,
~ ptr_eq v1 v2 ->
is_pointer_or_null p ->
P |-- list_token sh v1 * list_cell ls sh h v1 *
(field_at sh list_struct (StructField list_link :: nil)
(valinject (nested_field_type list_struct (StructField list_link :: nil)) p) v1 *
lseg ls sh tail p v2) ->
P |-- lseg ls sh (h::tail) v1 v2.
Lemma lseg_neq (ls: listspec list_structid list_link list_token):
forall sh s v v2,
ptr_neq v v2 ->
lseg ls sh s v v2 = lseg_cons ls sh s v v2.