Instance Join_trace : Join (AV.address -> option (rshare * AV.kind)) :=
     (Join_fun AV.address (option (rshare * AV.kind))
                   (Join_lower (Join_prod rshare Join_rshare AV.kind (Join_equiv AV.kind)))).

(* Specifications: *)
Lemma res_option_join: forall x y z, join x y z -> @join _ (@Join_lower (rshare * AV.kind) (Join_prod rshare Join_rshare AV.kind (Join_equiv AV.kind))) (res_option x) (res_option y) (res_option z).