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(* Copyright (c) 2012-2013, Robbert Krebbers. *)
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(* This file is distributed under the terms of the BSD license. *)
(** This file collects definitions and theorems on collections. Most
importantly, it implements some tactics to automatically solve goals involving
collections. *)
Require Export base tactics orders.

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(** * Basic theorems *)
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Section simple_collection.
  Context `{SimpleCollection A C}.
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  Lemma elem_of_empty x : x    False.
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  Proof. split. apply not_elem_of_empty. done. Qed.
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  Lemma elem_of_union_l x X Y : x  X  x  X  Y.
  Proof. intros. apply elem_of_union. auto. Qed.
  Lemma elem_of_union_r x X Y : x  Y  x  X  Y.
  Proof. intros. apply elem_of_union. auto. Qed.

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  Global Instance collection_subseteq: SubsetEq C := λ X Y,
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     x, x  X  x  Y.
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  Global Instance: BoundedJoinSemiLattice C.
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  Proof. firstorder auto. Qed.
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  Lemma elem_of_subseteq X Y : X  Y   x, x  X  x  Y.
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  Proof. done. Qed.
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  Lemma elem_of_equiv X Y : X  Y   x, x  X  x  Y.
  Proof. firstorder. Qed.
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  Lemma elem_of_equiv_alt X Y :
    X  Y  ( x, x  X  x  Y)  ( x, x  Y  x  X).
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  Proof. firstorder. Qed.
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  Lemma elem_of_equiv_empty X : X     x, x  X.
  Proof. firstorder. Qed.

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  Lemma elem_of_subseteq_singleton x X : x  X  {[ x ]}  X.
  Proof.
    split.
    * intros ??. rewrite elem_of_singleton. intro. by subst.
    * intros Ex. by apply (Ex x), elem_of_singleton.
  Qed.
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  Global Instance singleton_proper : Proper ((=) ==> ()) singleton.
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  Proof. repeat intro. by subst. Qed.
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  Global Instance elem_of_proper: Proper ((=) ==> () ==> iff) () | 5.
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  Proof. intros ???. subst. firstorder. Qed.

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  Lemma elem_of_union_list Xs x : x   Xs   X, X  Xs  x  X.
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  Proof.
    split.
    * induction Xs; simpl; intros HXs.
      + by apply elem_of_empty in HXs.
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      + setoid_rewrite elem_of_cons.
        apply elem_of_union in HXs. naive_solver.
    * intros [X []]. induction 1; simpl.
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      + by apply elem_of_union_l.
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      + intros. apply elem_of_union_r; auto.
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  Qed.

  Lemma non_empty_singleton x : {[ x ]}  .
  Proof. intros [E _]. by apply (elem_of_empty x), E, elem_of_singleton. Qed.

  Lemma not_elem_of_singleton x y : x  {[ y ]}  x  y.
  Proof. by rewrite elem_of_singleton. Qed.
  Lemma not_elem_of_union x X Y : x  X  Y  x  X  x  Y.
  Proof. rewrite elem_of_union. tauto. Qed.

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  Section leibniz.
    Context `{!LeibnizEquiv C}.

    Lemma elem_of_equiv_L X Y : X = Y   x, x  X  x  Y.
    Proof. unfold_leibniz. apply elem_of_equiv. Qed.
    Lemma elem_of_equiv_alt_L X Y :
      X = Y  ( x, x  X  x  Y)  ( x, x  Y  x  X).
    Proof. unfold_leibniz. apply elem_of_equiv_alt. Qed.
    Lemma elem_of_equiv_empty_L X : X =    x, x  X.
    Proof. unfold_leibniz. apply elem_of_equiv_empty. Qed.
    Lemma non_empty_singleton_L x : {[ x ]}  .
    Proof. unfold_leibniz. apply non_empty_singleton. Qed.
  End leibniz.

  Section dec.
    Context `{ X Y : C, Decision (X  Y)}.

    Global Instance elem_of_dec_slow (x : A) (X : C) : Decision (x  X) | 100.
    Proof.
      refine (cast_if (decide_rel () {[ x ]} X));
        by rewrite elem_of_subseteq_singleton.
    Defined.
  End dec.
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End simple_collection.

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(** * Tactics *)
(** Given a hypothesis [H : _ ∈ _], the tactic [destruct_elem_of H] will
recursively split [H] for [(∪)], [(∩)], [(∖)], [map], [∅], [{[_]}]. *)
Tactic Notation "decompose_elem_of" hyp(H) :=
  let rec go H :=
  lazymatch type of H with
  | _   => apply elem_of_empty in H; destruct H
  | ?x  {[ ?y ]} =>
    apply elem_of_singleton in H; try first [subst y | subst x]
  | _  _  _ =>
    let H1 := fresh in let H2 := fresh in apply elem_of_union in H;
    destruct H as [H1|H2]; [go H1 | go H2]
  | _  _  _ =>
    let H1 := fresh in let H2 := fresh in apply elem_of_intersection in H;
    destruct H as [H1 H2]; go H1; go H2
  | _  _  _ =>
    let H1 := fresh in let H2 := fresh in apply elem_of_difference in H;
    destruct H as [H1 H2]; go H1; go H2
  | ?x  _ <$> _ =>
    let H1 := fresh in apply elem_of_fmap in H;
    destruct H as [? [? H1]]; try (subst x); go H1
  | _  _ = _ =>
    let H1 := fresh in let H2 := fresh in apply elem_of_bind in H;
    destruct H as [? [H1 H2]]; go H1; go H2
  | ?x  mret ?y =>
    apply elem_of_ret in H; try first [subst y | subst x]
  | _  mjoin _ = _ =>
    let H1 := fresh in let H2 := fresh in apply elem_of_join in H;
    destruct H as [? [H1 H2]]; go H1; go H2
  | _ => idtac
  end in go H.
Tactic Notation "decompose_elem_of" :=
  repeat_on_hyps (fun H => decompose_elem_of H).

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Ltac decompose_empty := repeat
  match goal with
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  | H :    |- _ => clear H
  | H :  =  |- _ => clear H
  | H :   _ |- _ => symmetry in H
  | H :  = _ |- _ => symmetry in H
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  | H : _  _   |- _ => apply empty_union in H; destruct H
  | H : _  _   |- _ => apply non_empty_union in H; destruct H
  | H : {[ _ ]}   |- _ => destruct (non_empty_singleton _ H)
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  | H : _  _ =  |- _ => apply empty_union_L in H; destruct H
  | H : _  _   |- _ => apply non_empty_union_L in H; destruct H
  | H : {[ _ ]} =  |- _ => destruct (non_empty_singleton_L _ H)
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  end.

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(** The first pass of our collection tactic consists of eliminating all
occurrences of [(∪)], [(∩)], [(∖)], [(<$>)], [∅], [{[_]}], [(≡)], and [(⊆)],
by rewriting these into logically equivalent propositions. For example we
rewrite [A → x ∈ X ∪ ∅] into [A → x ∈ X ∨ False]. *)
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Ltac unfold_elem_of :=
  repeat_on_hyps (fun H =>
    repeat match type of H with
    | context [ _  _ ] => setoid_rewrite elem_of_subseteq in H
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    | context [ _  _ ] => setoid_rewrite subset_spec in H
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    | context [ _   ] => setoid_rewrite elem_of_equiv_empty in H
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    | context [ _  _ ] => setoid_rewrite elem_of_equiv_alt in H
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    | context [ _ =  ] => setoid_rewrite elem_of_equiv_empty_L in H
    | context [ _ = _ ] => setoid_rewrite elem_of_equiv_alt_L in H
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    | context [ _   ] => setoid_rewrite elem_of_empty in H
    | context [ _  {[ _ ]} ] => setoid_rewrite elem_of_singleton in H
    | context [ _  _  _ ] => setoid_rewrite elem_of_union in H
    | context [ _  _  _ ] => setoid_rewrite elem_of_intersection in H
    | context [ _  _  _ ] => setoid_rewrite elem_of_difference in H
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    | context [ _  _ <$> _ ] => setoid_rewrite elem_of_fmap in H
    | context [ _  mret _ ] => setoid_rewrite elem_of_ret in H
    | context [ _  _ = _ ] => setoid_rewrite elem_of_bind in H
    | context [ _  mjoin _ ] => setoid_rewrite elem_of_join in H
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    end);
  repeat match goal with
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  | |- context [ _  _ ] => setoid_rewrite elem_of_subseteq
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  | |- context [ _  _ ] => setoid_rewrite subset_spec
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  | |- context [ _   ] => setoid_rewrite elem_of_equiv_empty
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  | |- context [ _  _ ] => setoid_rewrite elem_of_equiv_alt
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  | |- context [ _ =  ] => setoid_rewrite elem_of_equiv_empty_L
  | |- context [ _ = _ ] => setoid_rewrite elem_of_equiv_alt_L
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  | |- context [ _   ] => setoid_rewrite elem_of_empty
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  | |- context [ _  {[ _ ]} ] => setoid_rewrite elem_of_singleton
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  | |- context [ _  _  _ ] => setoid_rewrite elem_of_union
  | |- context [ _  _  _ ] => setoid_rewrite elem_of_intersection
  | |- context [ _  _  _ ] => setoid_rewrite elem_of_difference
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  | |- context [ _  _ <$> _ ] => setoid_rewrite elem_of_fmap
  | |- context [ _  mret _ ] => setoid_rewrite elem_of_ret
  | |- context [ _  _ = _ ] => setoid_rewrite elem_of_bind
  | |- context [ _  mjoin _ ] => setoid_rewrite elem_of_join
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  end.

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(** The tactic [solve_elem_of tac] composes the above tactic with [intuition].
For goals that do not involve [≡], [⊆], [map], or quantifiers this tactic is
generally powerful enough. This tactic either fails or proves the goal. *)
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Tactic Notation "solve_elem_of" tactic3(tac) :=
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  simpl in *;
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  decompose_empty;
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  unfold_elem_of;
  solve [intuition (simplify_equality; tac)].
Tactic Notation "solve_elem_of" := solve_elem_of auto.

(** For goals with quantifiers we could use the above tactic but with
[firstorder] instead of [intuition] as finishing tactic. However, [firstorder]
fails or loops on very small goals generated by [solve_elem_of] already. We
use the [naive_solver] tactic as a substitute. This tactic either fails or
proves the goal. *)
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Tactic Notation "esolve_elem_of" tactic3(tac) :=
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  simpl in *;
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  decompose_empty;
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  unfold_elem_of;
  naive_solver tac.
Tactic Notation "esolve_elem_of" := esolve_elem_of eauto.
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(** * More theorems *)
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Section collection.
  Context `{Collection A C}.

  Global Instance: LowerBoundedLattice C.
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  Proof.
    split.
    * apply _.
    * firstorder auto.
    * solve_elem_of.
  Qed.
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  Lemma intersection_singletons x : {[x]}  {[x]}  {[x]}.
  Proof. esolve_elem_of. Qed.
  Lemma difference_twice X Y : (X  Y)  Y  X  Y.
  Proof. esolve_elem_of. Qed.

  Lemma empty_difference X Y : X  Y  X  Y  .
  Proof. esolve_elem_of. Qed.
  Lemma difference_diag X : X  X  .
  Proof. esolve_elem_of. Qed.
  Lemma difference_union_distr_l X Y Z : (X  Y)  Z  X  Z  Y  Z.
  Proof. esolve_elem_of. Qed.
  Lemma difference_intersection_distr_l X Y Z : (X  Y)  Z  X  Z  Y  Z.
  Proof. esolve_elem_of. Qed.

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  Section leibniz.
    Context `{!LeibnizEquiv C}.

    Lemma intersection_singletons_L x : {[x]}  {[x]} = {[x]}.
    Proof. unfold_leibniz. apply intersection_singletons. Qed.
    Lemma difference_twice_L X Y : (X  Y)  Y = X  Y.
    Proof. unfold_leibniz. apply difference_twice. Qed.

    Lemma empty_difference_L X Y : X  Y  X  Y = .
    Proof. unfold_leibniz. apply empty_difference. Qed.
    Lemma difference_diag_L X : X  X = .
    Proof. unfold_leibniz. apply difference_diag. Qed.
    Lemma difference_union_distr_l_L X Y Z : (X  Y)  Z = X  Z  Y  Z.
    Proof. unfold_leibniz. apply difference_union_distr_l. Qed.
    Lemma difference_intersection_distr_l_L X Y Z :
      (X  Y)  Z = X  Z  Y  Z.
    Proof. unfold_leibniz. apply difference_intersection_distr_l. Qed.
  End leibniz.

  Section dec.
    Context `{ X Y : C, Decision (X  Y)}.

    Lemma not_elem_of_intersection x X Y : x  X  Y  x  X  x  Y.
    Proof.
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      rewrite elem_of_intersection. destruct (decide (x  X)); tauto.
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    Qed.
    Lemma not_elem_of_difference x X Y : x  X  Y  x  X  x  Y.
    Proof.
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      rewrite elem_of_difference. destruct (decide (x  Y)); tauto.
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    Qed.
    Lemma union_difference X Y : X  Y  Y  X  Y  X.
    Proof.
      split; intros x; rewrite !elem_of_union, elem_of_difference.
      * destruct (decide (x  X)); intuition.
      * intuition.
    Qed.
    Lemma non_empty_difference X Y : X  Y  Y  X  .
    Proof.
      intros [HXY1 HXY2] Hdiff. destruct HXY2. intros x.
      destruct (decide (x  X)); esolve_elem_of.
    Qed.

    Context `{!LeibnizEquiv C}.

    Lemma union_difference_L X Y : X  Y  Y = X  Y  X.
    Proof. unfold_leibniz. apply union_difference. Qed.
    Lemma non_empty_difference_L X Y : X  Y  Y  X  .
    Proof. unfold_leibniz. apply non_empty_difference. Qed.
  End dec.
End collection.

Section collection_ops.
  Context `{CollectionOps A C}.

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  Lemma elem_of_intersection_with_list (f : A  A  option A) Xs Y x :
    x  intersection_with_list f Y Xs   xs y,
      Forall2 () xs Xs  y  Y  foldr (λ x, (= f x)) (Some y) xs = Some x.
  Proof.
    split.
    * revert x. induction Xs; simpl; intros x HXs.
      + eexists [], x. intuition.
      + rewrite elem_of_intersection_with in HXs.
        destruct HXs as (x1 & x2 & Hx1 & Hx2 & ?).
        destruct (IHXs x2) as (xs & y & hy & ? & ?); trivial.
        eexists (x1 :: xs), y. intuition (simplify_option_equality; auto).
    * intros (xs & y & Hxs & ? & Hx). revert x Hx.
      induction Hxs; intros; simplify_option_equality; [done |].
      rewrite elem_of_intersection_with. naive_solver.
  Qed.

  Lemma intersection_with_list_ind (P Q : A  Prop) f Xs Y :
    ( y, y  Y  P y) 
    Forall (λ X,  x, x  X  Q x) Xs 
    ( x y z, Q x  P y  f x y = Some z  P z) 
     x, x  intersection_with_list f Y Xs  P x.
  Proof.
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    intros HY HXs Hf. induction Xs; simplify_option_equality; [done |].
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    intros x Hx. rewrite elem_of_intersection_with in Hx.
    decompose_Forall. destruct Hx as (? & ? & ? & ? & ?). eauto.
  Qed.
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End collection_ops.
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(** * Sets without duplicates up to an equivalence *)
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Section NoDup.
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  Context `{SimpleCollection A B} (R : relation A) `{!Equivalence R}.
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  Definition elem_of_upto (x : A) (X : B) :=  y, y  X  R x y.
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  Definition set_NoDup (X : B) :=  x y, x  X  y  X  R x y  x = y.
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  Global Instance: Proper (() ==> iff) (elem_of_upto x).
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  Proof. intros ??? E. unfold elem_of_upto. by setoid_rewrite E. Qed.
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  Global Instance: Proper (R ==> () ==> iff) elem_of_upto.
  Proof.
    intros ?? E1 ?? E2. split; intros [z [??]]; exists z.
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    * rewrite <-E1, <-E2; intuition.
    * rewrite E1, E2; intuition.
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  Qed.
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  Global Instance: Proper (() ==> iff) set_NoDup.
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  Proof. firstorder. Qed.

  Lemma elem_of_upto_elem_of x X : x  X  elem_of_upto x X.
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  Proof. unfold elem_of_upto. esolve_elem_of. Qed.
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  Lemma elem_of_upto_empty x : ¬elem_of_upto x .
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  Proof. unfold elem_of_upto. esolve_elem_of. Qed.
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  Lemma elem_of_upto_singleton x y : elem_of_upto x {[ y ]}  R x y.
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  Proof. unfold elem_of_upto. esolve_elem_of. Qed.
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  Lemma elem_of_upto_union X Y x :
    elem_of_upto x (X  Y)  elem_of_upto x X  elem_of_upto x Y.
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  Proof. unfold elem_of_upto. esolve_elem_of. Qed.
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  Lemma not_elem_of_upto x X : ¬elem_of_upto x X   y, y  X  ¬R x y.
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  Proof. unfold elem_of_upto. esolve_elem_of. Qed.
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  Lemma set_NoDup_empty: set_NoDup .
  Proof. unfold set_NoDup. solve_elem_of. Qed.
  Lemma set_NoDup_add x X :
    ¬elem_of_upto x X  set_NoDup X  set_NoDup ({[ x ]}  X).
  Proof. unfold set_NoDup, elem_of_upto. esolve_elem_of. Qed.
  Lemma set_NoDup_inv_add x X :
    x  X  set_NoDup ({[ x ]}  X)  ¬elem_of_upto x X.
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  Proof.
    intros Hin Hnodup [y [??]].
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    rewrite (Hnodup x y) in Hin; solve_elem_of.
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  Qed.
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  Lemma set_NoDup_inv_union_l X Y : set_NoDup (X  Y)  set_NoDup X.
  Proof. unfold set_NoDup. solve_elem_of. Qed.
  Lemma set_NoDup_inv_union_r X Y : set_NoDup (X  Y)  set_NoDup Y.
  Proof. unfold set_NoDup. solve_elem_of. Qed.
End NoDup.
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(** * Quantifiers *)
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Section quantifiers.
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  Context `{SimpleCollection A B} (P : A  Prop).
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  Definition set_Forall X :=  x, x  X  P x.
  Definition set_Exists X :=  x, x  X  P x.

  Lemma set_Forall_empty : set_Forall .
  Proof. unfold set_Forall. solve_elem_of. Qed.
  Lemma set_Forall_singleton x : set_Forall {[ x ]}  P x.
  Proof. unfold set_Forall. solve_elem_of. Qed.
  Lemma set_Forall_union X Y : set_Forall X  set_Forall Y  set_Forall (X  Y).
  Proof. unfold set_Forall. solve_elem_of. Qed.
  Lemma set_Forall_union_inv_1 X Y : set_Forall (X  Y)  set_Forall X.
  Proof. unfold set_Forall. solve_elem_of. Qed.
  Lemma set_Forall_union_inv_2 X Y : set_Forall (X  Y)  set_Forall Y.
  Proof. unfold set_Forall. solve_elem_of. Qed.

  Lemma set_Exists_empty : ¬set_Exists .
  Proof. unfold set_Exists. esolve_elem_of. Qed.
  Lemma set_Exists_singleton x : set_Exists {[ x ]}  P x.
  Proof. unfold set_Exists. esolve_elem_of. Qed.
  Lemma set_Exists_union_1 X Y : set_Exists X  set_Exists (X  Y).
  Proof. unfold set_Exists. esolve_elem_of. Qed.
  Lemma set_Exists_union_2 X Y : set_Exists Y  set_Exists (X  Y).
  Proof. unfold set_Exists. esolve_elem_of. Qed.
  Lemma set_Exists_union_inv X Y :
    set_Exists (X  Y)  set_Exists X  set_Exists Y.
  Proof. unfold set_Exists. esolve_elem_of. Qed.
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End quantifiers.

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Section more_quantifiers.
  Context `{Collection A B}.
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  Lemma set_Forall_weaken (P Q : A  Prop) (Hweaken :  x, P x  Q x) X :
    set_Forall P X  set_Forall Q X.
  Proof. unfold set_Forall. naive_solver. Qed.
  Lemma set_Exists_weaken (P Q : A  Prop) (Hweaken :  x, P x  Q x) X :
    set_Exists P X  set_Exists Q X.
  Proof. unfold set_Exists. naive_solver. Qed.
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End more_quantifiers.

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(** * Fresh elements *)
(** We collect some properties on the [fresh] operation. In particular we
generalize [fresh] to generate lists of fresh elements. *)
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Section fresh.
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  Context `{FreshSpec A C} .
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  Definition fresh_sig (X : C) : { x : A | x  X } :=
    exist ( X) (fresh X) (is_fresh X).

  Global Instance fresh_proper: Proper (() ==> (=)) fresh.
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  Proof. intros ???. by apply fresh_proper_alt, elem_of_equiv. Qed.
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  Fixpoint fresh_list (n : nat) (X : C) : list A :=
    match n with
    | 0 => []
    | S n => let x := fresh X in x :: fresh_list n ({[ x ]}  X)
    end.

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  Global Instance fresh_list_proper: Proper ((=) ==> () ==> (=)) fresh_list.
  Proof.
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    intros ? n ?. subst. induction n; simpl; intros ?? E; f_equal.
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    * by rewrite E.
    * apply IHn. by rewrite E.
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  Qed.

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  Lemma fresh_list_length n X : length (fresh_list n X) = n.
  Proof. revert X. induction n; simpl; auto. Qed.

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  Lemma fresh_list_is_fresh n X x : x  fresh_list n X  x  X.
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  Proof.
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    revert X. induction n; intros X; simpl.
    * by rewrite elem_of_nil.
    * rewrite elem_of_cons. intros [?| Hin]; subst.
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      + apply is_fresh.
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      + apply IHn in Hin. solve_elem_of.
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  Qed.

  Lemma fresh_list_nodup n X : NoDup (fresh_list n X).
  Proof.
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    revert X. induction n; simpl; constructor; auto.
    intros Hin. apply fresh_list_is_fresh in Hin. solve_elem_of.
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  Qed.
End fresh.
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Definition option_collection `{Singleton A C} `{Empty C} (x : option A) : C :=
  match x with
  | None => 
  | Some a => {[ a ]}
  end.

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(** * Properties of implementations of collections that form a monad *)
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Section collection_monad.
  Context `{CollectionMonad M}.

  Global Instance collection_guard: MGuard M := λ P dec A x,
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    match dec with
    | left H => x H
    | _ => 
    end.
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  Global Instance collection_fmap_proper {A B} (f : A  B) :
    Proper (() ==> ()) (fmap f).
  Proof. intros X Y E. esolve_elem_of. Qed.
  Global Instance collection_ret_proper {A} :
    Proper ((=) ==> ()) (@mret M _ A).
  Proof. intros X Y E. esolve_elem_of. Qed.
  Global Instance collection_bind_proper {A B} (f : A  M B) :
    Proper (() ==> ()) (mbind f).
  Proof. intros X Y E. esolve_elem_of. Qed.
  Global Instance collection_join_proper {A} :
    Proper (() ==> ()) (@mjoin M _ A).
  Proof. intros X Y E. esolve_elem_of. Qed.

  Lemma collection_fmap_compose {A B C} (f : A  B) (g : B  C) X :
    g  f <$> X  g <$> (f <$> X).
  Proof. esolve_elem_of. Qed.
  Lemma elem_of_fmap_1 {A B} (f : A  B) (X : M A) (y : B) :
    y  f <$> X   x, y = f x  x  X.
  Proof. esolve_elem_of. Qed.
  Lemma elem_of_fmap_2 {A B} (f : A  B) (X : M A) (x : A) :
    x  X  f x  f <$> X.
  Proof. esolve_elem_of. Qed.
  Lemma elem_of_fmap_2_alt {A B} (f : A  B) (X : M A) (x : A) (y : B) :
    x  X  y = f x  y  f <$> X.
  Proof. esolve_elem_of. Qed.

  Lemma elem_of_mapM {A B} (f : A  M B) l k :
    l  mapM f k  Forall2 (λ x y, x  f y) l k.
  Proof.
    split.
    * revert l. induction k; esolve_elem_of.
    * induction 1; esolve_elem_of.
  Qed.
  Lemma mapM_length {A B} (f : A  M B) l k :
    l  mapM f k  length l = length k.
  Proof. revert l; induction k; esolve_elem_of. Qed.

  Lemma elem_of_mapM_fmap {A B} (f : A  B) (g : B  M A) l k :
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    Forall (λ x,  y, y  g x  f y = x) l  k  mapM g l  fmap f k = l.
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  Proof.
    intros Hl. revert k.
    induction Hl; simpl; intros;
      decompose_elem_of; simpl; f_equal; auto.
  Qed.

  Lemma elem_of_mapM_Forall {A B} (f : A  M B) (P : B  Prop) l k :
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    l  mapM f k  Forall (λ x,  y, y  f x  P y) k  Forall P l.
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  Proof. rewrite elem_of_mapM. apply Forall2_Forall_l. Qed.
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  Lemma elem_of_mapM_Forall2_l {A B C} (f : A  M B) (P: B  C  Prop) l1 l2 k :
    l1  mapM f k  Forall2 (λ x y,  z, z  f x  P z y) k l2 
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    Forall2 P l1 l2.
  Proof.
    rewrite elem_of_mapM. intros Hl1. revert l2.
    induction Hl1; inversion_clear 1; constructor; auto.
  Qed.
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End collection_monad.