(** Elememt [a] is a subelement of [c] .  This relation forms a partial order. *)
  Definition join_sub {A} `{Join A} (a c : A) : Prop :=
    exists b, join a b c.

Lemma join_join_sub {A} `{Perm_alg A}: forall {a b c}, join a b c -> join_sub a c.