Fixpoint extendr {A} (l : list A) ls :=
match l, ls with
| x :: xs, y :: ys => (y ++ [x]) :: extendr xs ys
| _, _ => ls
end.

Lemma list_join_eq : forall (b : list share) a c c'
(Hc : sepalg_list.list_join a b c) (Hc' : sepalg_list.list_join a b c'), c = c'.