Definition store_byte_list m b ofs lv :=
Mem.storebytes m b ofs (concat (map (encode_val Mint8unsigned) lv)).

Lemma change_ext : forall {Z} (a a' z : Z) (b c : ghost),
join [Some (ext_ghost a, NoneP)] b c ->
joins c [Some (ext_ref z, NoneP)] ->
join [Some (ext_ghost a', NoneP)] b (Some (ext_ghost a', NoneP) :: tl c).