Inductive sum_worlds {worlds1 worlds2}: Type:
    Type:=
  | lw (w:worlds1): sum_worlds
  | rw (w:worlds2): sum_worlds.Inductive sum_worlds {worlds1 worlds2}: Type:
    Type:=
  | lw (w:worlds1): sum_worlds
  | rw (w:worlds2): sum_worlds.Inductive sum_worlds {worlds1 worlds2}: Type:
    Type:=
  | lw (w:worlds1): sum_worlds
  | rw (w:worlds2): sum_worlds.Inductive sum_join {A B: Type} {J1: Join A} {J2: Join B}:
    @sum_worlds A B ->
    @sum_worlds A B ->
    @sum_worlds A B-> Prop :=
  | left_join a b c:
      join a b c ->
      sum_join (lw a) (lw b) (lw c)
  | right_join a b c:
      join a b c ->
      sum_join (rw a) (rw b) (rw c).Inductive sum_join {A B: Type} {J1: Join A} {J2: Join B}:
    @sum_worlds A B ->
    @sum_worlds A B ->
    @sum_worlds A B-> Prop :=
  | left_join a b c:
      join a b c ->
      sum_join (lw a) (lw b) (lw c)
  | right_join a b c:
      join a b c ->
      sum_join (rw a) (rw b) (rw c).Inductive sum_join {A B: Type} {J1: Join A} {J2: Join B}:
    @sum_worlds A B ->
    @sum_worlds A B ->
    @sum_worlds A B-> Prop :=
  | left_join a b c:
      join a b c ->
      sum_join (lw a) (lw b) (lw c)
  | right_join a b c:
      join a b c ->
      sum_join (rw a) (rw b) (rw c).Definition sum_Join (A B: Type) {Join_A: Join A} {Join_B: Join B}: Join (@sum_worlds A B) :=
    (@sum_join A B Join_A Join_B).Definition sum_SA (A B: Type) {Join_A: Join A} {Join_B: Join B} {SA_A: SeparationAlgebra A} {SA_B: SeparationAlgebra B}: @SeparationAlgebra (@sum_worlds A B) (sum_Join A B).Definition prod_Join (A B: Type) {Join_A: Join A} {Join_B: Join B}: Join (A * B) :=
    (fun a b c => join (fst a) (fst b) (fst c) /\ join (snd a) (snd b) (snd c)).Definition prod_SA (A B: Type) {Join_A: Join A} {Join_B: Join B} {SA_A: SeparationAlgebra A} {SA_B: SeparationAlgebra B}: @SeparationAlgebra (A * B) (prod_Join A B).

Lemma prod_unitalSA (A B: Type) {R_A: Relation A} {R_B: Relation B} {Join_A: Join A} {Join_B: Join B} {unitalSA_A: UnitalSeparationAlgebra A} {unitalSA_B: UnitalSeparationAlgebra B}: @UnitalSeparationAlgebra (A * B) (RelProd R_A R_B) (@prod_Join _ _ Join_A Join_B).