Instance unit_PCM : Ghost := { valid a := True; Join_G a b c := True }.

Lemma agree_join2 : forall g P1 P2, agree g P1 * agree g P2 |-- (|> P1 -* |> P2) * agree g P2.
Inductive list_join {P : Ghost} : Join (list (option G)) :=
| list_join_nil_l m: list_join nil m m
| list_join_nil_r m: list_join m nil m
| list_join_cons a1 a2 m1 m2 a3 m3: join a1 a2 a3 -> list_join m1 m2 m3 ->
list_join (a1 :: m1) (a2 :: m2) (a3 :: m3).