Global Set Implicit Arguments.
Global Unset Strict Implicit.


Require Export Omega List Morphisms.


Ltac inv H := inversion H; subst; clear H.

Lemma DM_or (X Y : Prop) :
  ¬ (X ∨ Y) ↔ ¬ X ∧ ¬ Y.
Proof.
  tauto.
Qed.

Lemma DM_exists X (p : X → Prop) :
  ¬ (∃ x, p x) ↔ ∀ x, ¬ p x.
Proof.
  firstorder.
Qed.

Lemma size_recursion (X : Type) (sigma : X → nat) (p : X → Type) :
  (∀ x, (∀ y, sigma y < sigma x → p y) → p x) →
  ∀ x, p x.
Proof.
  intros D x. apply D.
  cut (∀ n y, sigma y < n → p y).
  now eauto.
  clear x. intros n.
  induction n; intros y E.
  - exfalso; omega.
  - apply D. intros x F. apply IHn. omega.
Qed.

Arguments size_recursion {X} sigma {p} _ _.

Section Iteration.
  Variable X : Type.
  Variable f : X → X.

  Fixpoint it (n : nat) (x : X) : X :=
    match n with
      | 0 ⇒ x
      | S n' ⇒ f (it n' x)
    end.

  Lemma it_ind (p : X → Prop) x n :
    p x → (∀ z, p z → p (f z)) → p (it n x).
  Proof.
    intros A B. induction n; simpl; auto.
  Qed.

  Definition FP (x : X) : Prop := f x = x.

  Lemma it_fp (sigma : X → nat) x :
    (∀ n, FP (it n x) ∨ sigma (it n x) > sigma (it (S n) x)) →
    FP (it (sigma x) x).
  Proof.
    intros A.
    assert (B: ∀ n, FP (it n x) ∨ sigma x ≥ n + sigma (it n x)).
    { intros n; induction n; simpl.
      - auto.
      - destruct IHn as [B|B].
        + left. now rewrite B.
        + destruct (A n) as [C|C].
          × left. now rewrite C.
          × right. simpl in C. omega. }
    destruct (A (sigma x)), (B (sigma x)); auto; exfalso; omega.
  Qed.
End Iteration.

Definition dec (X : Prop) : Type := {X} + {¬ X}.

Notation "'eq_dec' X" := (∀ x y : X, dec (x=y)) (at level 70).


Existing Class dec.

Definition decision (X : Prop) (D : dec X) : dec X := D.
Arguments decision X {D}.

Tactic Notation "decide" constr(p) :=
  destruct (decision p).
Tactic Notation "decide" constr(p) "as" simple_intropattern(i) :=
  destruct (decision p) as i.


Hint Extern 4 ⇒
match goal with
  | [ |- dec ?p ] ⇒ exact (decision p)
end.


Hint Extern 4 ⇒
match goal with
  | [ |- dec ((fun _ ⇒ _) _) ] ⇒ simpl
end : typeclass_instances.


Instance True_dec : dec True :=
  left I.

Instance False_dec : dec False :=
  right (fun A ⇒ A).

Instance impl_dec (X Y : Prop) :
  dec X → dec Y → dec (X → Y).
Proof.
  unfold dec; tauto.
Defined.

Instance and_dec (X Y : Prop) :
  dec X → dec Y → dec (X ∧ Y).
Proof.
  unfold dec; tauto.
Defined.

Instance or_dec (X Y : Prop) :
  dec X → dec Y → dec (X ∨ Y).
Proof.
  unfold dec; tauto.
Defined.


Instance not_dec (X : Prop) :
  dec X → dec (¬ X).
Proof.
  unfold not. auto.
Defined.

Instance iff_dec (X Y : Prop) :
  dec X → dec Y → dec (X ↔ Y).
Proof.
  unfold iff. auto.
Qed.

Lemma dec_DN X :
  dec X → ~~ X → X.
Proof.
  unfold dec; tauto.
Qed.

Lemma dec_DM_and X Y :
  dec X → dec Y → ¬ (X ∧ Y) → ¬ X ∨ ¬ Y.
Proof.
  unfold dec; tauto.
Qed.

Lemma dec_DM_impl X Y :
  dec X → dec Y → ¬ (X → Y) → X ∧ ¬ Y.
Proof.
  unfold dec; tauto.
Qed.

Lemma dec_prop_iff (X Y : Prop) :
  (X ↔ Y) → dec X → dec Y.
Proof.
  unfold dec; tauto.
Defined.

Instance bool_eq_dec :
  eq_dec bool.
Proof.
  intros x y. hnf. decide equality.
Defined.

Instance nat_eq_dec :
  eq_dec nat.
Proof.
  intros x y. hnf. decide equality.
Defined.

Instance nat_le_dec (x y : nat) : dec (x ≤ y) :=
  le_dec x y.

Definition equi X (A B : list X) : Prop :=
  incl A B ∧ incl B A.

Hint Unfold equi.

Export ListNotations.
Notation "| A |" := (length A) (at level 65).
Notation "x 'el' A" := (In x A) (at level 70).
Notation "A <<= B" := (incl A B) (at level 70).
Notation "A === B" := (equi A B) (at level 70).

Hint Rewrite <- app_assoc : list.
Hint Rewrite rev_app_distr map_app prod_length : list.

Lemma list_cycle (X : Type) (A : list X) x :
  x::A ≠ A.
Proof.
  intros B.
  assert (C: |x::A| ≠ |A|) by (simpl; omega).
  apply C. now rewrite B.
Qed.

Instance list_eq_dec X :
  eq_dec X → eq_dec (list X).
Proof.
  intros D. apply list_eq_dec. exact D.
Defined.

Instance list_in_dec (X : Type) (x : X) (A : list X) :
  eq_dec X → dec (x \u220a A).
Proof.
  intros D. apply in_dec. exact D.
Defined.

Lemma list_sigma_forall X A (p : X → Prop) (p_dec : ∀ x, dec (p x)) :
  {x | x \u220a A ∧ p x} + {∀ x, x \u220a A → ¬ p x}.
Proof.
  induction A as [|x A]; simpl.
  - tauto.
  - destruct IHA as [[y [D E]]|D].
    + eauto.
    + destruct (p_dec x) as [E|E].
      × eauto.
      × right. intros y [[]|F]; auto.
Defined.

Arguments list_sigma_forall {X} A p {p_dec}.

Instance list_forall_dec X A (p : X → Prop) :
  (∀ x, dec (p x)) → dec (∀ x, x \u220a A → p x).
Proof.
  intros p_dec.
  destruct (list_sigma_forall A (fun x ⇒ ¬ p x)) as [[x [D E]]|D].
  - right. auto.
  - left. intros x E. apply dec_DN; auto.
Defined.

Instance list_exists_dec X A (p : X → Prop) :
  (∀ x, dec (p x)) → dec (∃ x, x \u220a A ∧ p x).
Proof.
  intros p_dec.
  destruct (list_sigma_forall A p) as [[x [D E]]|D].
  - unfold dec. eauto.
  - right. intros [x [E F]]. exact (D x E F).
Defined.

Lemma list_exists_DM X A (p : X → Prop) :
  (∀ x, dec (p x)) →
  ¬ (∀ x, x \u220a A → ¬ p x) → ∃ x, x \u220a A ∧ p x.
Proof.
  intros D E.
  destruct (list_sigma_forall A p) as [F|F].
  + destruct F as [x F]. eauto.
  + contradiction (E F).
Qed.

Lemma list_exists_not_incl X (A B : list X) :
  eq_dec X →
  ¬ A \u2286 B → ∃ x, x \u220a A ∧ ¬ x \u220a B.
Proof.
  intros D E.
  apply list_exists_DM; auto.
  intros F. apply E. intros x G.
  apply dec_DN; auto.
Qed.

Lemma list_cc X (p : X → Prop) A :
  (∀ x, dec (p x)) →
  (∃ x, x \u220a A ∧ p x) → {x | x \u220a A ∧ p x}.
Proof.
  intros D E.
  destruct (list_sigma_forall A p) as [[x [F G]]|F].
  - eauto.
  - exfalso. destruct E as [x [G H]]. apply (F x); auto.
Defined.

Hint Resolve in_eq in_nil in_cons in_or_app.

Section Membership.
  Variable X : Type.
  Implicit Types x y : X.
  Implicit Types A B : list X.

  Lemma in_sing x y :
    x \u220a [y] → x = y.
  Proof.
    simpl. intros [[]|[]]. reflexivity.
  Qed.

  Lemma in_cons_neq x y A :
    x \u220a y::A → x ≠ y → x \u220a A.
  Proof.
    simpl. intros [[]|D] E; congruence.
  Qed.

  Lemma not_in_cons x y A :
    ¬ x \u220a y :: A → x ≠ y ∧ ¬ x \u220a A.
  Proof.
    intuition; subst; auto.
  Qed.

  Definition disjoint A B :=
    ¬ ∃ x, x \u220a A ∧ x \u220a B.

  Lemma disjoint_forall A B :
    disjoint A B ↔ ∀ x, x \u220a A → ¬ x \u220a B.
  Proof.
    split.
    - intros D x E F. apply D. ∃ x. auto.
    - intros D [x [E F]]. exact (D x E F).
  Qed.

  Lemma disjoint_symm A B :
    disjoint A B → disjoint B A.
  Proof.
    firstorder.
  Qed.

  Lemma disjoint_incl A B B' :
    B' \u2286 B → disjoint A B → disjoint A B'.
  Proof.
    firstorder.
  Qed.

  Lemma disjoint_nil B :
    disjoint nil B.
  Proof.
    firstorder.
  Qed.

  Lemma disjoint_nil' A :
    disjoint A nil.
  Proof.
    firstorder.
  Qed.

  Lemma disjoint_cons x A B :
    disjoint (x::A) B ↔ ¬ x \u220a B ∧ disjoint A B.
  Proof.
    split.
    - intros D. split.
      + intros E. apply D. eauto.
      + intros [y [E F]]. apply D. eauto.
    - intros [D E] [y [[F|F] G]].
      + congruence.
      + apply E. eauto.
  Qed.

  Lemma disjoint_app A B C :
    disjoint (A ++ B) C ↔ disjoint A C ∧ disjoint B C.
  Proof.
    split.
    - intros D. split.
      + intros [x [E F]]. eauto 6.
      + intros [x [E F]]. eauto 6.
    - intros [D E] [x [F G]].
      apply in_app_iff in F as [F|F]; eauto.
  Qed.

End Membership.

Hint Resolve disjoint_nil disjoint_nil'.

Hint Resolve incl_refl incl_tl incl_cons incl_appl incl_appr incl_app.

Lemma incl_nil X (A : list X) :
  nil \u2286 A.

Proof. intros x []. Qed.

Hint Resolve incl_nil.

Lemma incl_map X Y A B (f : X → Y) :
  A \u2286 B → map f A \u2286 map f B.

Proof.
  intros D y E. apply in_map_iff in E as [x [E E']].
  subst y. apply in_map_iff. eauto.
Qed.

Section Inclusion.
  Variable X : Type.
  Implicit Types A B : list X.

  Lemma incl_nil_eq A :
    A \u2286 nil → A=nil.

  Proof.
    intros D. destruct A as [|x A].
    - reflexivity.
    - exfalso. apply (D x). auto.
  Qed.

  Lemma incl_shift x A B :
    A \u2286 B → x::A \u2286 x::B.

  Proof. auto. Qed.

  Lemma incl_lcons x A B :
    x::A \u2286 B ↔ x \u220a B ∧ A \u2286 B.
  Proof.
    split.
    - intros D. split; hnf; auto.
    - intros [D E] z [F|F]; subst; auto.
  Qed.

  Lemma incl_sing x A y :
    x::A \u2286 [y] → x = y ∧ A \u2286 [y].
  Proof.
    rewrite incl_lcons. intros [D E].
    apply in_sing in D. auto.
  Qed.

  Lemma incl_rcons x A B :
    A \u2286 x::B → ¬ x \u220a A → A \u2286 B.

  Proof. intros C D y E. destruct (C y E) as [F|F]; congruence. Qed.

  Lemma incl_lrcons x A B :
    x::A \u2286 x::B → ¬ x \u220a A → A \u2286 B.

  Proof.
    intros C D y E.
    assert (F: y \u220a x::B) by auto.
    destruct F as [F|F]; congruence.
  Qed.

  Lemma incl_app_left A B C :
    A ++ B \u2286 C → A \u2286 C ∧ B \u2286 C.
  Proof.
    firstorder.
  Qed.

End Inclusion.

Definition inclp (X : Type) (A : list X) (p : X → Prop) : Prop :=
  ∀ x, x \u220a A → p x.

Instance incl_preorder X :
  PreOrder (@incl X).
Proof.
  constructor; hnf; unfold incl; auto.
Qed.

Instance equi_Equivalence X :
  Equivalence (@equi X).
Proof.
  constructor; hnf; firstorder.
Qed.

Instance incl_equi_proper X :
  Proper (@equi X ==> @equi X ==> iff) (@incl X).
Proof.
  hnf. intros A B D. hnf. firstorder.
Qed.

Instance cons_incl_proper X x :
  Proper (@incl X ==> @incl X) (@cons X x).
Proof.
  hnf. apply incl_shift.
Qed.

Instance cons_equi_proper X x :
  Proper (@equi X ==> @equi X) (@cons X x).
Proof.
  hnf. firstorder.
Qed.

Instance in_incl_proper X x :
  Proper (@incl X ==> Basics.impl) (@In X x).
Proof.
  intros A B D. hnf. auto.
Qed.

Instance in_equi_proper X x :
  Proper (@equi X ==> iff) (@In X x).
Proof.
  intros A B D. firstorder.
Qed.

Instance app_incl_proper X :
  Proper (@incl X ==> @incl X ==> @incl X) (@app X).
Proof.
  intros A B D A' B' E. auto.
Qed.

Instance app_equi_proper X :
  Proper (@equi X ==> @equi X ==> @equi X) (@app X).
Proof.
  hnf. intros A B D. hnf. intros A' B' E.
  destruct D, E; auto.
Qed.

Section Equi.
  Variable X : Type.
  Implicit Types A B : list X.

  Lemma equi_push x A :
    x \u220a A → A \u2261 x::A.

  Proof. auto. Qed.

  Lemma equi_dup x A :
    x::A \u2261 x::x::A.

  Proof. auto. Qed.

  Lemma equi_swap x y A:
    x::y::A \u2261 y::x::A.

  Proof. split; intros z; simpl; tauto. Qed.

  Lemma equi_shift x A B :
    x::A++B \u2261 A++x::B.

  Proof.
    split; intros y.
    - intros [D|D].
      + subst; auto.
      + apply in_app_iff in D as [D|D]; auto.
    - intros D. apply in_app_iff in D as [D|D].
      + auto.
      + destruct D; subst; auto.
  Qed.

  Lemma equi_rotate x A :
    x::A \u2261 A++[x].

  Proof.
    split; intros y; simpl.
    - intros [D|D]; subst; auto.
    - intros D. apply in_app_iff in D as [D|D].
      + auto.
      + apply in_sing in D. auto.
  Qed.
End Equi.

Definition filter (X : Type) (p : X → Prop) (p_dec : ∀ x, dec (p x)) : list X → list X :=
  fix f A := match A with
              | nil ⇒ nil
              | x::A' ⇒ if decision (p x) then x :: f A' else f A'
            end.

Arguments filter {X} p {p_dec} A.

Section FilterLemmas.
  Variable X : Type.
  Variable p : X → Prop.
  Context {p_dec : ∀ x, dec (p x)}.

  Lemma in_filter_iff x A :
    x \u220a filter p A ↔ x \u220a A ∧ p x.

  Proof.
    induction A as [|y A]; simpl.
    - tauto.
    - decide (p y) as [B|B]; simpl;
      rewrite IHA; intuition; subst; tauto.
  Qed.

  Lemma filter_incl A :
    filter p A \u2286 A.

  Proof.
    intros x D. apply in_filter_iff in D. apply D.
  Qed.

  Lemma filter_mono A B :
    A \u2286 B → filter p A \u2286 filter p B.

  Proof.
    intros D x E. apply in_filter_iff in E as [E E'].
    apply in_filter_iff. auto.
  Qed.

  Lemma filter_id A :
    (∀ x, x \u220a A → p x) → filter p A = A.
  Proof.
    intros D.
    induction A as [|x A]; simpl.
    - reflexivity.
    - decide (p x) as [E|E].
      + f_equal; auto.
      + exfalso. auto.
  Qed.

  Lemma filter_app A B :
    filter p (A ++ B) = filter p A ++ filter p B.
  Proof.
    induction A as [|y A]; simpl.
    - reflexivity.
    - rewrite IHA. decide (p y); reflexivity.
  Qed.

  Lemma filter_fst x A :
    p x → filter p (x::A) = x::filter p A.
  Proof.
    simpl. decide (p x); tauto.
  Qed.

  Lemma filter_fst' x A :
    ¬ p x → filter p (x::A) = filter p A.
  Proof.
    simpl. decide (p x); tauto.
  Qed.

End FilterLemmas.

Section FilterLemmas_pq.
  Variable X : Type.
  Variable p q : X → Prop.
  Context {p_dec : ∀ x, dec (p x)}.
  Context {q_dec : ∀ x, dec (q x)}.

  Lemma filter_pq_mono A :
    (∀ x, x \u220a A → p x → q x) → filter p A \u2286 filter q A.

  Proof.
    intros D x E. apply in_filter_iff in E as [E E'].
    apply in_filter_iff. auto.
  Qed.

  Lemma filter_pq_eq A :
    (∀ x, x \u220a A → (p x ↔ q x)) → filter p A = filter q A.

  Proof.
    intros C; induction A as [|x A]; simpl.
    - reflexivity.
    - decide (p x) as [D|D]; decide (q x) as [E|E].
      + f_equal. auto.
      + exfalso. apply E, (C x); auto.
      + exfalso. apply D, (C x); auto.
      + auto.
  Qed.

  Lemma filter_and A :
    filter p (filter q A) = filter (fun x ⇒ p x ∧ q x) A.
  Proof.
    set (r x := p x ∧ q x).
    induction A as [|x A]; simpl. reflexivity.
    decide (q x) as [E|E]; simpl; rewrite IHA.
    - decide (p x); decide (r x); unfold r in *|-; auto; tauto.
    - decide (r x); unfold r in *|-; auto; tauto.
  Qed.

End FilterLemmas_pq.

Section FilterComm.
  Variable X : Type.
  Variable p q : X → Prop.
  Context {p_dec : ∀ x, dec (p x)}.
  Context {q_dec : ∀ x, dec (q x)}.

  Lemma filter_comm A :
    filter p (filter q A) = filter q (filter p A).
  Proof.
    rewrite !filter_and. apply filter_pq_eq. tauto.
  Qed.
End FilterComm.

Section Removal.
  Variable X : Type.
  Context {eq_X_dec : eq_dec X}.

  Definition rem (A : list X) (x : X) : list X :=
    filter (fun z ⇒ z ≠ x) A.

  Lemma in_rem_iff x A y :
    x \u220a rem A y ↔ x \u220a A ∧ x ≠ y.
  Proof.
    apply in_filter_iff.
  Qed.

  Lemma rem_not_in x y A :
    x = y ∨ ¬ x \u220a A → ¬ x \u220a rem A y.
  Proof.
    intros D E. apply in_rem_iff in E. tauto.
  Qed.

  Lemma rem_incl A x :
    rem A x \u2286 A.
  Proof.
    apply filter_incl.
  Qed.

  Lemma rem_mono A B x :
    A \u2286 B → rem A x \u2286 rem B x.
  Proof.
    apply filter_mono.
  Qed.

  Lemma rem_cons A B x :
    A \u2286 B → rem (x::A) x \u2286 B.
  Proof.
    intros E y F. apply E. apply in_rem_iff in F.
    destruct F as [[|]]; congruence.
  Qed.

  Lemma rem_cons' A B x y :
    x \u220a B → rem A y \u2286 B → rem (x::A) y \u2286 B.
  Proof.
    intros E F u G.
    apply in_rem_iff in G as [[[]|G] H]. exact E.
    apply F. apply in_rem_iff. auto.
  Qed.

  Lemma rem_in x y A :
    x \u220a rem A y → x \u220a A.
  Proof.
    apply rem_incl.
  Qed.

  Lemma rem_neq x y A :
    x ≠ y → x \u220a A → x \u220a rem A y.
  Proof.
    intros E F. apply in_rem_iff. auto.
  Qed.

  Lemma rem_app x A B :
    x \u220a A → B \u2286 A ++ rem B x.
  Proof.
    intros E y F. decide (x=y) as [[]|]; auto using rem_neq.
  Qed.

  Lemma rem_app' x A B C :
    rem A x \u2286 C → rem B x \u2286 C → rem (A ++ B) x \u2286 C.
  Proof.
    unfold rem; rewrite filter_app; auto.
  Qed.

  Lemma rem_equi x A :
    x::A \u2261 x::rem A x.
  Proof.
    split; intros y;
    intros [[]|E]; decide (x=y) as [[]|D];
    eauto using rem_in, rem_neq.
  Qed.

  Lemma rem_comm A x y :
    rem (rem A x) y = rem (rem A y) x.
  Proof.
    apply filter_comm.
  Qed.

  Lemma rem_fst x A :
    rem (x::A) x = rem A x.
  Proof.
    unfold rem. rewrite filter_fst'; auto.
  Qed.

  Lemma rem_fst' x y A :
    x ≠ y → rem (x::A) y = x::rem A y.
  Proof.
    intros E. unfold rem. rewrite filter_fst; auto.
  Qed.

  Lemma rem_id x A :
    ¬ x \u220a A → rem A x = A.
  Proof.
    intros D. apply filter_id.
    intros y E F. subst. auto.
  Qed.

  Lemma rem_reorder x A :
    x \u220a A → A \u2261 x :: rem A x.
  Proof.
    intros D. rewrite <- rem_equi. apply equi_push, D.
  Qed.

  Lemma rem_inclr A B x :
    A \u2286 B → ¬ x \u220a A → A \u2286 rem B x.
  Proof.
    intros D E y F. apply in_rem_iff.
    intuition; subst; auto.
  Qed.

End Removal.

Hint Resolve rem_not_in rem_incl rem_mono rem_cons rem_cons' rem_app rem_app' rem_in rem_neq rem_inclr.

Section Cardinality.
  Variable X : Type.
  Context { eq_X_dec : eq_dec X }.
  Implicit Types A B : list X.

  Fixpoint card A :=
    match A with
      | nil ⇒ 0
      | x::A ⇒ if decision (x \u220a A) then card A else 1 + card A
    end.

  Lemma card_in_rem x A :
    x \u220a A → card A = 1 + card (rem A x).
  Proof.
    intros D.
    induction A as [|y A]; simpl.
    - contradiction D.
    - decide (y ≠ x) as [E|E]; simpl.
      + rewrite IHA.
        × { decide (y \u220a A) as [G|G];
            decide (y \u220a rem A x) as [K|K];
            (reflexivity || exfalso).
            - auto.
            - eapply G, in_rem_iff, K. }
        × destruct D; tauto.
      + apply dec_DN in E; auto; subst y.
        decide (x \u220a A) as [F|F].
        × auto.
        × rewrite rem_id; auto.
  Qed.

  Lemma card_not_in_rem A x :
    ¬ x \u220a A → card A = card (rem A x).
  Proof.
    intros D; rewrite rem_id; auto.
  Qed.

  Lemma card_le A B :
    A \u2286 B → card A ≤ card B.
  Proof.
  revert B.
  induction A as [|x A]; intros B D; simpl.
  - omega.
  - apply incl_lcons in D as [D D1].
    decide (x \u220a A) as [E|E].
    + auto.
    + rewrite (card_in_rem D).
      cut (card A ≤ card (rem B x)). omega.
      apply IHA. auto.
  Qed.

  Lemma card_eq A B :
    A \u2261 B → card A = card B.
  Proof.
    intros [E F]. apply card_le in E. apply card_le in F. omega.
  Qed.

  Lemma card_cons_rem x A :
    card (x::A) = 1 + card (rem A x).
  Proof.
    rewrite (card_eq (rem_equi x A)). simpl.
    decide (x \u220a rem A x) as [D|D].
    - exfalso. apply in_rem_iff in D; tauto.
    - reflexivity.
  Qed.

  Lemma card_0 A :
    card A = 0 → A = nil.
  Proof.
    destruct A as [|x A]; intros D.
    - reflexivity.
    - exfalso. rewrite card_cons_rem in D. omega.
  Qed.

  Lemma card_ex A B :
    card A < card B → ∃ x, x \u220a B ∧ ¬ x \u220a A.
  Proof.
    intros D.
    decide (B \u2286 A) as [E|E].
    - exfalso. apply card_le in E. omega.
    - apply list_exists_not_incl; auto.
  Qed.

  Lemma card_equi A B :
    A \u2286 B → card A = card B → A \u2261 B.
  Proof.
    revert B.
    induction A as [|x A]; simpl; intros B D E.
    - symmetry in E. apply card_0 in E. now rewrite E.
    - apply incl_lcons in D as [D D1].
      decide (x \u220a A) as [F|F].
      + rewrite (IHA B); auto.
      + rewrite (IHA (rem B x)).
        × symmetry. apply rem_reorder, D.
        × auto.
        × apply card_in_rem in D. omega.
  Qed.

  Lemma card_lt A B x :
    A \u2286 B → x \u220a B → ¬ x \u220a A → card A < card B.
  Proof.
    intros D E F.
    decide (card A = card B) as [G|G].
    + exfalso. apply F. apply (card_equi D); auto.
    + apply card_le in D. omega.
  Qed.

  Lemma card_or A B :
    A \u2286 B → A \u2261 B ∨ card A < card B.
  Proof.
    intros D.
    decide (card A = card B) as [F|F].
    - left. apply card_equi; auto.
    - right. apply card_le in D. omega.
  Qed.

End Cardinality.

Instance card_equi_proper X (D: eq_dec X) :
  Proper (@equi X ==> eq) (@card X D).
Proof.
  hnf. apply card_eq.
Qed.

Inductive dupfree (X : Type) : list X → Prop :=
| dupfreeN : dupfree nil
| dupfreeC x A : ¬ x \u220a A → dupfree A → dupfree (x::A).

Section Dupfree.
  Variable X : Type.
  Implicit Types A B : list X.

  Lemma dupfree_cons x A :
    dupfree (x::A) ↔ ¬ x \u220a A ∧ dupfree A.
  Proof.
    split; intros D.
    - inv D; auto.
    - apply dupfreeC; tauto.
  Qed.

  Lemma dupfree_app A B :
    disjoint A B → dupfree A → dupfree B → dupfree (A++B).

  Proof.
    intros D E F. induction E as [|x A E' E]; simpl.
    - exact F.
    - apply disjoint_cons in D as [D D'].
      constructor; [|exact (IHE D')].
      intros G. apply in_app_iff in G; tauto.
  Qed.

  Lemma dupfree_map Y A (f : X → Y) :
    (∀ x y, x \u220a A → y \u220a A → f x = f y → x=y) →
    dupfree A → dupfree (map f A).

  Proof.
    intros D E. induction E as [|x A E' E]; simpl.
    - constructor.
    - constructor; [|now auto].
      intros F. apply in_map_iff in F as [y [F F']].
      rewrite (D y x) in F'; auto.
  Qed.

  Lemma dupfree_filter p (p_dec : ∀ x, dec (p x)) A :
    dupfree A → dupfree (filter p A).

  Proof.
    intros D. induction D as [|x A C D]; simpl.
    - left.
    - decide (p x) as [E|E]; [|exact IHD].
      right; [|exact IHD].
      intros F. apply C. apply filter_incl in F. exact F.
   Qed.

  Lemma dupfree_dec A :
    eq_dec X → dec (dupfree A).

  Proof.
    intros D. induction A as [|x A].
    - left. left.
    - decide (x \u220a A) as [E|E].
      + right. intros F. inv F; tauto.
      + destruct (IHA) as [F|F].
        × unfold dec. auto using dupfree.
        × right. intros G. inv G; tauto.
  Qed.

  Lemma dupfree_card A (eq_X_dec : eq_dec X) :
    dupfree A → card A = |A|.
  Proof.
    intros D.
    induction D as [|x A E F]; simpl.
    - reflexivity.
    - decide (x \u220a A) as [G|].
      + contradiction (E G).
      + omega.
  Qed.

End Dupfree.

Section Undup.
  Variable X : Type.
  Context {eq_X_dec : eq_dec X}.
  Implicit Types A B : list X.

  Fixpoint undup (A : list X) : list X :=
    match A with
      | nil ⇒ nil
      | x::A' ⇒ if decision (x \u220a A') then undup A' else x :: undup A'
    end.

  Lemma undup_id_equi A :
    undup A \u2261 A.
  Proof.
    induction A as [|x A]; simpl.
    - reflexivity.
    - decide (x \u220a A) as [E|E]; rewrite IHA; auto.
  Qed.

  Lemma dupfree_undup A :
    dupfree (undup A).
  Proof.
    induction A as [|x A]; simpl.
    - left.
    - decide (x \u220a A) as [E|E]; auto.
      right; auto. now rewrite undup_id_equi.
  Qed.

  Lemma undup_incl A B :
    A \u2286 B ↔ undup A \u2286 undup B.
  Proof.
    now rewrite !undup_id_equi.
  Qed.

  Lemma undup_equi A B :
    A \u2261 B ↔ undup A \u2261 undup B.
  Proof.
    now rewrite !undup_id_equi.
  Qed.

  Lemma undup_id A :
    dupfree A → undup A = A.
  Proof.
    intros E. induction E as [|x A E F]; simpl.
    - reflexivity.
    - rewrite IHF. decide (x \u220a A) as [G|G]; tauto.
  Qed.

  Lemma undup_idempotent A :
    undup (undup A) = undup A.

  Proof. apply undup_id, dupfree_undup. Qed.

End Undup.

Section PowerRep.
  Variable X : Type.
  Context {eq_X_dec : eq_dec X}.

  Fixpoint power (U : list X ) : list (list X) :=
    match U with
      | nil ⇒ [nil]
      | x :: U' ⇒ power U' ++ map (cons x) (power U')
    end.

  Lemma power_incl A U :
    A \u220a power U → A \u2286 U.

  Proof.
    revert A; induction U as [|x U]; simpl; intros A D.
    - destruct D as [[]|[]]; auto.
    - apply in_app_iff in D as [E|E]. now auto.
      apply in_map_iff in E as [A' [E F]]. subst A.
      auto.
  Qed.

  Lemma power_nil U :
    nil \u220a power U.

  Proof. induction U; simpl; auto. Qed.

  Definition rep (A U : list X) : list X :=
    filter (fun x ⇒ x \u220a A) U.

  Lemma rep_power A U :
    rep A U \u220a power U.

  Proof.
    revert A; induction U as [|x U]; intros A.
    - simpl; auto.
    - simpl. decide (x \u220a A) as [D|D]; auto using in_map.
  Qed.

  Lemma rep_incl A U :
    rep A U \u2286 A.

  Proof.
    unfold rep. intros x D. apply in_filter_iff in D. apply D.
  Qed.

  Lemma rep_in x A U :
    A \u2286 U → x \u220a A → x \u220a rep A U.
  Proof.
    intros D E. apply in_filter_iff; auto.
  Qed.

  Lemma rep_equi A U :
    A \u2286 U → rep A U \u2261 A.

  Proof.
    intros D. split. now apply rep_incl.
    intros x. apply rep_in, D.
  Qed.

  Lemma rep_mono A B U :
    A \u2286 B → rep A U \u2286 rep B U.

  Proof. intros D. apply filter_pq_mono. auto. Qed.

  Lemma rep_eq' A B U :
    (∀ x, x \u220a U → (x \u220a A ↔ x \u220a B)) → rep A U = rep B U.

  Proof. intros D. apply filter_pq_eq. auto. Qed.

  Lemma rep_eq A B U :
    A \u2261 B → rep A U = rep B U.

  Proof. intros D. apply filter_pq_eq. firstorder. Qed.

  Lemma rep_injective A B U :
    A \u2286 U → B \u2286 U → rep A U = rep B U → A \u2261 B.

  Proof.
    intros D E F. transitivity (rep A U).
    - symmetry. apply rep_equi, D.
    - rewrite F. apply rep_equi, E.
  Qed.

  Lemma rep_idempotent A U :
    rep (rep A U) U = rep A U.

  Proof.
    unfold rep at 1 3. apply filter_pq_eq.
    intros x D. split.
    + apply rep_incl.
    + intros E. apply in_filter_iff. auto.
  Qed.

  Lemma dupfree_power U :
    dupfree U → dupfree (power U).

  Proof.
    intros D. induction D as [|x U E D]; simpl.
    - constructor. now auto. constructor.
    - apply dupfree_app.
      + intros [A [F G]]. apply in_map_iff in G as [A' [G G']].
        subst A. apply E. apply (power_incl F). auto.
      + exact IHD.
      + apply dupfree_map; congruence.
  Qed.

  Lemma dupfree_in_power U A :
    A \u220a power U → dupfree U → dupfree A.

  Proof.
    intros E D. revert A E.
    induction D as [|x U D D']; simpl; intros A E.
    - destruct E as [[]|[]]. constructor.
    - apply in_app_iff in E as [E|E].
      + auto.
      + apply in_map_iff in E as [A' [E E']]. subst A.
        constructor.
        × intros F; apply D. apply (power_incl E'), F.
        × auto.
  Qed.

  Lemma rep_dupfree A U :
    dupfree U → A \u220a power U → rep A U = A.

  Proof.
    intros D; revert A.
    induction D as [|x U E F]; intros A G.
    - destruct G as [[]|[]]; reflexivity.
    - simpl in G. apply in_app_iff in G as [G|G].
      + simpl. decide (x \u220a A) as [H|H].
        × exfalso. apply E. apply (power_incl G), H.
        × auto.
      + apply in_map_iff in G as [A' [G H]]. subst A.
        simpl. decide (x=x ∨ x \u220a A') as [G|G].
        × f_equal. rewrite <- (IHF A' H) at 2.
          apply filter_pq_eq. apply power_incl in H.
          intros z K. split; [|now auto].
          intros [L|L]; subst; tauto.
        × exfalso; auto.
  Qed.

  Lemma power_extensional A B U :
    dupfree U → A \u220a power U → B \u220a power U → A \u2261 B → A = B.

  Proof.
    intros D E F G.
    rewrite <- (rep_dupfree D E). rewrite <- (rep_dupfree D F).
    apply rep_eq, G.
  Qed.

End PowerRep.

Module FCI.
Section FCI.
  Variable X : Type.
  Context {eq_X_dec : eq_dec X}.
  Variable step : list X → X → Prop.
  Context {step_dec : ∀ A x, dec (step A x)}.
  Variable V : list X.

  Lemma pick (A : list X) :
    { x | x \u220a V ∧ step A x ∧ ¬ x \u220a A } + { ∀ x, x \u220a V → step A x → x \u220a A }.
  Proof.
    destruct (list_sigma_forall V (fun x ⇒ step A x ∧ ¬ x \u220a A)) as [E|E].
    - auto.
    - right. intros x F G.
      decide (x \u220a A). assumption. exfalso.
      eapply E; eauto.
  Qed.

  Definition F (A : list X) : list X.
    destruct (pick A) as [[x _]|_].
    - exact (x::A).
    - exact A.
  Defined.

  Definition C := it F (card V) nil.

  Lemma it_incl n :
    it F n nil \u2286 V.
  Proof.
    apply it_ind. now auto.
    intros A E. unfold F.
    destruct (pick A) as [[x G]|G]; intuition.
  Qed.

  Lemma incl :
    C \u2286 V.
  Proof.
    apply it_incl.
  Qed.

  Lemma ind p :
    (∀ A x, inclp A p → x \u220a V → step A x → p x) → inclp C p.
  Proof.
    intros B. unfold C. apply it_ind.
    + intros x [].
    + intros D G x. unfold F.
      destruct (pick D) as [[y E]|E].
      × intros [[]|]; intuition; eauto.
      × intuition.
  Qed.

  Lemma fp :
    F C = C.
  Proof.
    pose (sigma (A : list X) := card V - card A).
    replace C with (it F (sigma nil) nil).
    - apply it_fp. intros n. simpl.
      set (J:= it F n nil). unfold FP, F.
      destruct (pick J) as [[x B]|B].
      + right.
        assert (G: card J < card (x :: J)).
        { apply card_lt with (x:=x); intuition. }
        assert (H: card (x :: J) ≤ card V).
        { apply card_le, incl_cons. apply B. apply it_incl. }
        unfold sigma. omega.
      + auto.
    - unfold C, sigma. f_equal. change (card nil) with 0. omega.
  Qed.

  Lemma closure x :
    x \u220a V → step C x → x \u220a C.
  Proof.
    assert (A2:= fp).
    unfold F in A2.
    destruct (pick C) as [[y _]| B].
    + contradiction (list_cycle A2).
    + apply B.
  Qed.

End FCI.
End FCI.