Definition lseg_cons (ls: listspec list_structid list_link list_token) dsh psh
(l: list val) (x z: val) : mpred :=
!! (~ ptr_eq x z) &&
EX h:(elemtype ls), EX r:list val, EX y:val,
!!(l=x::r  /\ is_pointer_or_null y) &&
list_token dsh x * list_cell ls dsh 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.

Lemma lseg_unroll (ls: listspec list_structid list_link list_token): forall dsh psh l x z ,
~ (readable_share dsh) ->
lseg ls dsh psh l x z =
(!! (ptr_eq x z) && !! (l=nil) && emp) || lseg_cons ls dsh psh l x z.