Definition resource_at (phi:rmap) : address -> resource := proj1_sig (snd (unsquash phi)).
Infix "@" := resource_at (at level 50, no associativity).
Instance Join_nat_rmap': Join (nat * rmap') := Join_prod _ (Join_equiv nat) _ _.

(* Specifications: *)
Axiom join_unsquash : forall phi1 phi2 phi3,
join phi1 phi2 phi3 <->
join (unsquash phi1) (unsquash phi2) (unsquash phi3).