Library Coq.Sets.Ensembles

Section Ensembles.
  Variable U : Type.

  Definition Ensemble := U -> Prop.

  Definition In (A:Ensemble) (x:U) : Prop := A x.

  Definition Included (B C:Ensemble) : Prop := forall x:U, In B x -> In C x.

  Inductive Empty_set : Ensemble :=.

  Inductive Full_set : Ensemble :=
    Full_intro : forall x:U, In Full_set x.

NB: The following definition builds-in equality of elements in U as Leibniz equality.

This may have to be changed if we replace U by a Setoid on U with its own equality eqs, with In_singleton: (y: U)(eqs x y) -> (In (Singleton x) y).

  Inductive Singleton (x:U) : Ensemble :=
    In_singleton : In (Singleton x) x.

  Inductive Union (B C:Ensemble) : Ensemble :=
    | Union_introl : forall x:U, In B x -> In (Union B C) x
    | Union_intror : forall x:U, In C x -> In (Union B C) x.

  Definition Add (B:Ensemble) (x:U) : Ensemble := Union B (Singleton x).

  Inductive Intersection (B C:Ensemble) : Ensemble :=
    Intersection_intro :
    forall x:U, In B x -> In C x -> In (Intersection B C) x.

  Inductive Couple (x y:U) : Ensemble :=
    | Couple_l : In (Couple x y) x
    | Couple_r : In (Couple x y) y.

  Inductive Triple (x y z:U) : Ensemble :=
    | Triple_l : In (Triple x y z) x
    | Triple_m : In (Triple x y z) y
    | Triple_r : In (Triple x y z) z.

  Definition Complement (A:Ensemble) : Ensemble := fun x:U => ~ In A x.

  Definition Setminus (B C:Ensemble) : Ensemble :=
    fun x:U => In B x /\ ~ In C x.

  Definition Subtract (B:Ensemble) (x:U) : Ensemble := Setminus B (Singleton x).

  Inductive Disjoint (B C:Ensemble) : Prop :=
    Disjoint_intro : (forall x:U, ~ In (Intersection B C) x ) -> Disjoint B C.

  Inductive Inhabited (B:Ensemble) : Prop :=
    Inhabited_intro : forall x:U, In B x -> Inhabited B.

  Definition Strict_Included (B C:Ensemble) : Prop := Included B C /\ B <> C.

  Definition Same_set (B C:Ensemble) : Prop := Included B C /\ Included C B.

Extensionality Axiom

  Axiom Extensionality_Ensembles : forall A B:Ensemble, Same_set A B -> A = B.

End Ensembles.

#[global]
Hint Unfold In Included Same_set Strict_Included Add Setminus Subtract: sets.

#[global]
Hint Resolve Union_introl Union_intror Intersection_intro In_singleton
  Couple_l Couple_r Triple_l Triple_m Triple_r Disjoint_intro
  Extensionality_Ensembles: sets.