Class IncreasingJoinSelfSeparationAlgebra
(worlds: Type)
{R: Relation worlds}
{J: Join worlds}: Type :=
incr_join_self:
forall m, increasing m -> join m m m.
Class IncreasingSplitSmallerSeparationAlgebra
(worlds: Type)
{R: Relation worlds}
{J: Join worlds}: Type :=
incr_split_smaller:
forall m1 m2 m, increasing m -> join m1 m2 m -> m1 <= m.
Class UpwardsClosedSeparationAlgebra
(worlds: Type)
{R: Relation worlds}
{J: Join worlds}: Type :=
join_Korder_up: forall m n m1 m2: worlds,
join m1 m2 m -> m <= n ->
exists n1 n2, join n1 n2 n /\ m1 <= n1 /\ m2 <= n2.
Class DownwardsClosedSeparationAlgebra
(worlds: Type)
{R: Relation worlds}
{J: Join worlds}: Type :=
join_Korder_down: forall m1 m2 m n1 n2: worlds,
join m1 m2 m -> n1 <= m1 -> n2 <= m2 ->
exists n, join n1 n2 n /\ n <= m.
(* David J. Pym, Peter W. O’Hearn, and Hongseok Yang. Possible worlds and resources: the semantics of BI. *)
(* This join_Korder is equivalent with the following because Korder is reflexive. join_Korder: forall M (m1 m2 m n1 : Kworlds M), join m1 m2 m -> Korder m1 n1 -> exists n, join n1 m2 n /\ Korder m n;  It is necessary to be this strong, or else sepcon_assoc will be unsound, e.g. the following weaker version causes unsoundness: join_Korder: forall M (m1 m2 m n1: Kworlds M), join m1 m2 m -> Korder m1 n1 -> exists n2 n, join n1 n2 n /\ Korder m2 n2 /\ Korder m n;  *)

Lemma residual_extensible
{worlds: Type}
{R: Relation worlds}
{po_R: PreOrder Krelation}
{J: Join worlds}
{SA: SeparationAlgebra worlds}
{dSA: DownwardsClosedSeparationAlgebra worlds}
{resSA: ResidualSeparationAlgebra worlds}:
forall u, exists e v, join u e v.