Definition lseg_cons (ls: listspec list_structid list_link list_token) dsh psh (l: list (val * elemtype ls)) (x z: val) : mpred :=
!! (~ ptr_eq x z) &&
EX h:(elemtype ls), EX r:list (val * elemtype ls), EX y:val,
!!(l=(x,h)::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_neq (ls: listspec list_structid list_link list_token):
forall dsh psh s v v2,
ptr_neq v v2 ->
lseg ls dsh psh s v v2 = lseg_cons ls dsh psh s v v2.