(* Sing_alg: adds the property that makes the Permission Algebra
     into a single-unit Separation Algebra.  (this is the "traditional"
     kind of Separation Algebra).   *)
Class Sing_alg A {J: Join A}{SA: Sep_alg A} :=
    mkSing {
      the_unit: A;
      the_unit_core: forall a, core a = the_unit
    }.(* Sing_alg: adds the property that makes the Permission Algebra
     into a single-unit Separation Algebra.  (this is the "traditional"
     kind of Separation Algebra).   *)
Class Sing_alg A {J: Join A}{SA: Sep_alg A} :=
    mkSing {
      the_unit: A;
      the_unit_core: forall a, core a = the_unit
    }.(* Sing_alg: adds the property that makes the Permission Algebra
     into a single-unit Separation Algebra.  (this is the "traditional"
     kind of Separation Algebra).   *)
Class Sing_alg A {J: Join A}{SA: Sep_alg A} :=
    mkSing {
      the_unit: A;
      the_unit_core: forall a, core a = the_unit
    }.(* Sing_alg: adds the property that makes the Permission Algebra
     into a single-unit Separation Algebra.  (this is the "traditional"
     kind of Separation Algebra).   *)
Class Sing_alg A {J: Join A}{SA: Sep_alg A} :=
    mkSing {
      the_unit: A;
      the_unit_core: forall a, core a = the_unit
    }.(* Positive Permission Algebra: there are no units, every element is nonempty *)
  Class Pos_alg  {A} {J: Join A} :=
    no_units: forall e a, ~unit_for e a.(* Positive Permission Algebra: there are no units, every element is nonempty *)
  Class Pos_alg  {A} {J: Join A} :=
    no_units: forall e a, ~unit_for e a.(* Has the "cross-split" property described in Dockins et al,
    "A fresh look at separation algebras and share accounting", 2009 *)
Class Cross_alg (t: Type)  `{J: Join t} :=
   cross_split :
      forall a b c d z : t,
       join a b z ->
       join c d z ->
    { x:(t*t*t*t) &  match x with (ac,ad,bc,bd) =>
         join ac ad a /\ join bc bd b /\ join ac bc c /\ join ad bd d
       end
    }.(* Has the "cross-split" property described in Dockins et al,
    "A fresh look at separation algebras and share accounting", 2009 *)
Class Cross_alg (t: Type)  `{J: Join t} :=
   cross_split :
      forall a b c d z : t,
       join a b z ->
       join c d z ->
    { x:(t*t*t*t) &  match x with (ac,ad,bc,bd) =>
         join ac ad a /\ join bc bd b /\ join ac bc c /\ join ad bd d
       end
    }.(* Has the "triple join" property  *)
Class Trip_alg {A} {J: Join A} :=
  triple_join_exists:
  forall (a b c ab bc ac : A), join a b ab -> join b c bc -> join a c ac ->
       {abc | join ab c abc}.(* Has the "triple join" property  *)
Class Trip_alg {A} {J: Join A} :=
  triple_join_exists:
  forall (a b c ab bc ac : A), join a b ab -> join b c bc -> join a c ac ->
       {abc | join ab c abc}.

Lemma join_ex_units{A}{J: Join A}{SA: Sep_alg A}: forall a, {e : A & unit_for e a }.