Definition lseg_cell  (ls: listspec list_structid list_link list_token)
(dsh psh : share)
(v: elemtype ls) (x y: val) :=
!!is_pointer_or_null y && list_token dsh x * list_cell ls dsh v x * field_at psh list_struct (StructField list_link :: nil) (valinject (nested_field_type list_struct (StructField list_link :: nil)) y) x.

Lemma list_append_null:
forall
(ls: listspec list_structid list_link list_token)
(dsh psh: share)
(hd mid: val) ct1 ct2,
lseg ls dsh psh ct1 hd mid * lseg ls dsh psh ct2 mid nullval |--
lseg ls dsh psh (ct1++ct2) hd nullval.