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.

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.