Definition same_constructive_silhouette {A} {JOIN: Join A} (a b: A) :=
    forall c, (constructive_joins c a -> constructive_joins c b) *
                 (constructive_joins c b -> constructive_joins c a).Definition sub_constructive_silhouette {A} {JOIN: Join A}  (a b: A) :=
    forall c, constructive_joins c b -> constructive_joins c a.

Lemma same_constructive_silhouette_sym {A} {JOIN: Join A}: forall a b, same_constructive_silhouette a b -> same_constructive_silhouette b a.