Global Set Implicit Arguments.
Global Unset Strict Implicit.


Require Export Omega List Morphisms.


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

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 Unfold dec.
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.

Instance prod_eq_dec X Y (HX: eq_dec X) (HY : eq_dec Y): eq_dec (X × Y).
Proof with try now (constructor;congruence).
  intros [x1 y1] [x2 y2].
  decide (x1=x2)... decide (y1=y2)...
Defined.

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).



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 ∊ 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 ∊ A ∧ p x} + {∀ x, x ∊ 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) (p_dec : ∀ x, dec (p x)) :
  dec (∀ x, x ∊ A → p x).
Proof.
  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) (p_dec : ∀ x, dec (p x)) :
  dec (∃ x, x ∊ A ∧ p x).
Proof.
  destruct (list_sigma_forall A p) as [[x [D E]]|D].
  - 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 ∊ A → ¬ p x) → ∃ x, x ∊ 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_cc X (p : X → Prop) A :
  (∀ x, dec (p x)) →
  (∃ x, x ∊ A ∧ p x) → {x | x ∊ 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.

Lemma in_sing X (x y : X) :
  x ∊ [y] → x = y.

Proof. simpl. intros [[]|[]]. reflexivity. Qed.

Lemma in_cons_neq X (x y : X) A :
  x ∊ y::A → x ≠ y → x ∊ A.

Proof. simpl. intros [[]|D] E; congruence. Qed.

Definition disjoint (X : Type) (A B : list X) :=
  ¬ ∃ x, x ∊ A ∧ x ∊ B.

Lemma disjoint_forall X (A B : list X) :
  disjoint A B ↔ ∀ x, x ∊ A → ¬ x ∊ 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_cons X (x : X) A B :
  disjoint (x::A) B ↔ ¬ x ∊ 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.

Hint Resolve incl_refl incl_tl incl_cons incl_appl incl_appr incl_app.

Lemma incl_nil X (A : list X) :
  nil ⊆ A.

Proof. intros x []. Qed.

Hint Resolve incl_nil.

Lemma incl_map X Y A B (f : X → Y) :
  A ⊆ B → map f A ⊆ 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 ⊆ 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 ⊆ B → x::A ⊆ x::B.

  Proof. intros D y E. destruct E; subst; auto. Qed.

  Lemma incl_lcons x A B :
    x::A ⊆ B ↔ x ∊ B ∧ A ⊆ B.

  Proof.
    split.
    - intros D. split; hnf; auto.
    - intros [D E] z [F|F]; subst; auto.
  Qed.

  Lemma incl_rcons x A B :
    A ⊆ x::B → ¬ x ∊ A → A ⊆ B.

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

  Lemma incl_lrcons x A B :
    x::A ⊆ x::B → ¬ x ∊ A → A ⊆ B.

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

End Inclusion.

Hint Resolve incl_shift.

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

Instance in_equi_proper X :
  Proper (eq ==> @equi X ==> iff) (@In X).

Proof. hnf. intros x y []. hnf. firstorder. Qed.

Instance incl_equi_proper X :
  Proper (@equi X ==> @equi X ==> iff) (@incl X).

Proof. hnf. intros x y D. hnf. firstorder. Qed.

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 cons_equi_proper X :
  Proper (eq ==> @equi X ==> @equi X) (@cons X).

Proof. hnf. intros x y []. hnf. firstorder. 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 ∊ A → A ≡ x::A.

  Proof. auto. Qed.

  Lemma equi_dup x A :
    x::A ≡ x::x::A.

  Proof. auto. Qed.

  Lemma equi_swap x y A:
    x::y::A ≡ y::x::A.

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

  Lemma equi_shift x A B :
    x::A++B ≡ 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 ≡ 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 ∊ filter p A ↔ x ∊ 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 ⊆ A.

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

  Lemma filter_mono A B :
    A ⊆ B → filter p A ⊆ 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_fst x A :
    p x → filter p (x::A) = x::filter p A.
  Proof.
    simpl. decide (p x); tauto.
  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) = 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 ∊ A → p x → q x) → filter p A ⊆ 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 ∊ 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.
    do 2 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 ∊ rem A y ↔ x ∊ A ∧ x ≠ y.
  Proof.
    apply in_filter_iff.
  Qed.

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

  Lemma rem_incl A x :
    rem A x ⊆ A.
  Proof.
    apply filter_incl.
  Qed.

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

  Lemma rem_cons A B x :
    A ⊆ B → rem (x::A) x ⊆ 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 ∊ B → rem A y ⊆ B → rem (x::A) y ⊆ 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 ∊ rem A y → x ∊ A.
  Proof.
    apply rem_incl.
  Qed.

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

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

  Lemma rem_equi x A :
    x::A ≡ 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.

End Removal.

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

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

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

  Lemma dupfree_inv x A :
    dupfree (x::A) ↔ ¬ x ∊ 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 ∊ A → y ∊ 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 ∊ A) as [E|E].
      + right. intros F. inv F; tauto.
      + destruct (IHA) as [F|F].
        × auto using dupfree.
        × right. intros G. inv G; tauto.
  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 ∊ A') then undup A' else x :: undup A'
    end.

  Lemma undup_fp_equi A :
    undup A ≡ A.
  Proof.
    induction A as [|x A]; simpl.
    - reflexivity.
    - decide (x ∊ 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 ∊ A) as [E|E]; auto.
      right; auto. now rewrite undup_fp_equi.
  Qed.

  Lemma undup_incl A B :
    A ⊆ B ↔ undup A ⊆ undup B.
  Proof.
    now do 2 rewrite undup_fp_equi.
  Qed.

  Lemma undup_equi A B :
    A ≡ B ↔ undup A ≡ undup B.
  Proof.
    now do 2 rewrite undup_fp_equi.
  Qed.

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

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

  Proof. apply undup_eq, dupfree_undup. Qed.

End Undup.

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

  Lemma dupfree_reorder A x :
    dupfree A → x ∊ A →
    ∃ A', A ≡ x::A' ∧ |A'| < |A| ∧ dupfree (x::A').

  Proof.
    intros E. revert x. induction E as [|y A H]; intros x F.
    - contradiction F.
    - destruct F as [F|F].
      + subst y. ∃ A. auto using dupfree.
      + specialize (IHE x F). destruct IHE as [A' [G [K1 K2]]].
        ∃ (y::A'). split; [|split].
        × rewrite G. apply equi_swap.
        × simpl. omega.
        × { apply dupfree_inv in K2 as [K2 K3]. right.
            - intros [M|M]; subst; auto.
            - right; [|exact K3].
              intros M; apply H. apply G. auto. }
   Qed.

  Lemma dupfree_le A B :
    dupfree A → dupfree B → A ⊆ B → |A| ≤ |B|.

  Proof.
    intros E; revert B.
    induction A as [|x A]; simpl; intros B F G.
    - omega.
    - apply incl_lcons in G as [G H].
      destruct (dupfree_reorder F G) as [B' [K [L M]]].
      apply dupfree_inv in E as [E1 E2].
      apply dupfree_inv in M as [M1 M2].
      cut (A ⊆ B').
      { intros N. specialize (IHA E2 B' M2 N). omega. }
      apply incl_rcons with (x:=x); [|exact E1].
      rewrite H. apply K.
  Qed.

  Lemma dupfree_eq A B :
    dupfree A → dupfree B → A ≡ B → |A|=|B|.

  Proof.
    intros D E [F G].
    apply (dupfree_le D E) in F.
    apply (dupfree_le E D) in G.
    omega.
  Qed.

  Lemma dupfree_lt A B x :
    dupfree A → dupfree B → A ⊆ B →
    x ∊ B → ¬ x ∊ A → |A| < |B|.

  Proof.
    intros E F G H K.
    destruct (dupfree_reorder F H) as [B' [L [M N]]].
    rewrite (dupfree_eq F N L).
    cut (|A|≤|B'|). { simpl; omega. }
    apply dupfree_le.
    - exact E.
    - now inv N.
    - apply incl_rcons with (x:=x).
      + rewrite G. apply L.
      + exact K.
  Qed.

  Lemma dupfree_ex A B :
    eq_dec X → dupfree A → dupfree B → |A| < |B| → ∃ x, x ∊ B ∧ ¬ x ∊ A.

  Proof.
    intros D E F H.
    destruct (list_sigma_forall B (fun x ⇒ ¬ x ∊ A)) as [[x K]|K].
    - ∃ x; exact K.
    - exfalso.
      assert (L : B ⊆ A).
      { intros x L. apply dec_DN; auto. }
      apply dupfree_le in L; auto; omega.
  Qed.

  Lemma dupfree_equi A B :
    eq_dec X → dupfree A → dupfree B → A ⊆ B → |A|=|B| → A ≡ B.

  Proof.
    intros C D E F G. split. exact F.
    destruct (list_sigma_forall B (fun x ⇒ ¬ x ∊ A)) as [[x [H K]]|H].
    - exfalso. assert (L:=dupfree_lt D E F H K). omega.
    - intros x L. apply dec_DN; auto.
  Qed.

End DupfreeLength.

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

  Definition card (A : list X) : nat := |undup A|.

  Lemma card_le A B :
    A ⊆ B → card A ≤ card B.

  Proof.
    intros E. apply dupfree_le.
    - apply dupfree_undup.
    - apply dupfree_undup.
    - apply undup_incl, E.
  Qed.

  Lemma card_eq A B :
    A ≡ B → card A = card B.

  Proof.
    intros [E F]. apply card_le in E. apply card_le in F. omega.
  Qed.

  Lemma card_equi A B :
    A ⊆ B → card A = card B → A ≡ B.
  Proof.
    intros D E.
    apply <- undup_equi. apply → undup_incl in D.
    apply dupfree_equi; auto using dupfree_undup.
  Qed.

  Lemma card_lt A B x :
    A ⊆ B → x ∊ B → ¬ x ∊ A → card A < card B.

  Proof.
    intros D E F.
    apply (dupfree_lt (A:= undup A) (B:= undup B) (x:=x)).
    - apply dupfree_undup.
    - apply dupfree_undup.
    - apply undup_incl, D.
    - apply undup_fp_equi, E.
    - rewrite undup_fp_equi. exact F.
  Qed.

  Lemma card_or A B :
    A ⊆ B → A ≡ 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.

  Lemma card_ex A B :
    card A < card B → ∃ x, x ∊ B ∧ ¬ x ∊ A.

  Proof.
    intros E.
    destruct (dupfree_ex (A:=undup A) (B:=undup B)) as [x F].
    - exact eq_X_dec.
    - apply dupfree_undup.
    - apply dupfree_undup.
    - exact E.
    - ∃ x. setoid_rewrite undup_fp_equi in F. exact F.
  Qed.

  Lemma card_cons x A :
    card (x::A) = if decision (x ∊ A) then card A else 1 + card A.
  Proof.
    unfold card at 1; simpl. now decide (x ∊ A).
  Qed.

  Lemma card_cons_rem x A :
    card (x::A) = 1 + card (rem A x).
  Proof.
    rewrite (card_eq (rem_equi x A)).
    rewrite card_cons.
    decide (x ∊ rem A x) as [D|D].
    - 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.

End Cardinality.

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

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 ∊ power U → A ⊆ 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.
      apply incl_shift. auto.
  Qed.

  Lemma power_nil U :
    nil ∊ power U.

  Proof. induction U; simpl; auto. Qed.

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

  Lemma rep_power A U :
    rep A U ∊ power U.

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

  Lemma rep_incl A U :
    rep A U ⊆ A.

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

  Lemma rep_in x A U :
    A ⊆ U → x ∊ A → x ∊ rep A U.
  Proof.
    intros D E. apply in_filter_iff; auto.
  Qed.

  Lemma rep_equi A U :
    A ⊆ U → rep A U ≡ A.

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

  Lemma rep_mono A B U :
    A ⊆ B → rep A U ⊆ rep B U.

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

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

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

  Lemma rep_eq A B U :
    A ≡ B → rep A U = rep B U.

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

  Lemma rep_injective A B U :
    A ⊆ U → B ⊆ U → rep A U = rep B U → A ≡ 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 ∊ 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 ∊ 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 ∊ 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 ∊ 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 ∊ power U → B ∊ power U → A ≡ 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 ∊ V ∧ step A x ∧ ¬ x ∊ A } + { ∀ x, x ∊ V → step A x → x ∊ A }.
  Proof.
    destruct (list_sigma_forall V (fun x ⇒ step A x ∧ ¬ x ∊ A)) as [E|E].
    - auto.
    - right. intros x F G.
      decide (x ∊ 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 ⊆ V.
  Proof.
    apply it_ind. now auto.
    intros A E. unfold F.
    destruct (pick A) as [[x G]|G]; intuition.
  Qed.

  Lemma incl :
    C ⊆ V.
  Proof.
    apply it_incl.
  Qed.

  Lemma ind p :
    (∀ A x, inclp A p → x ∊ 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 ∊ V → step C x → x ∊ 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.

Lemma complete_induction (p : nat → Type) (x : nat) :
(∀ x, (∀ y, y<x → p y) → p x) → p x.

Proof. intros A. apply A. induction x ; intros y B.
exfalso ; omega.
apply A. intros z C. apply IHx. omega. Qed.

Lemma size_induction X (f : X → nat) (p : X → Type) :
  (∀ x, (∀ y, f y < f x → p y) → p x) →
  ∀ x, p x.

Proof.
  intros IH x. apply IH.
  assert (G: ∀ n y, f y < n → p y).
  { intros n. induction n.
    - intros y B. exfalso. omega.
    - intros y B. apply IH. intros z C. apply IHn. omega. }
  apply G.
Qed.

Section pos.

  Definition elAt := nth_error.
  Notation "A '.[' i ']'" := (elAt A i) (no associativity, at level 50).

  Variable X:Type.
  Context {Eq_dec:eq_dec X}.
  Fixpoint pos (s : X) (A : list X) :=
    match A with
      | nil ⇒ None
      | a :: A ⇒ if decision (s = a) then Some 0 else match pos s A with None ⇒ None | Some n ⇒ Some (S n) end
    end.

  Lemma el_pos s A : s ∊ A → ∃ m, pos s A = Some m.
  Proof.
    revert s; induction A; simpl; intros s H.
      - contradiction.
      - decide (s = a) as [D | D]; eauto;
        destruct H; try congruence.
        destruct (IHA s H) as [n Hn]; eexists; now rewrite Hn.
  Qed.

  Lemma pos_elAt s A i : pos s A = Some i → A .[i] = Some s.
  Proof.
    revert i s. induction A; intros i s.
    - destruct i; inversion 1.
    - simpl. decide (s = a).
      + inversion 1; subst; reflexivity.
      + destruct i; destruct (pos s A) eqn:B; inversion 1; subst; eauto.
  Qed.

  Lemma elAt_app (A : list X) i B s : A .[i] = Some s → (A ++ B).[i] = Some s.
  Proof.
    revert s B i. induction A; intros s B i H; destruct i; simpl; intuition; inv H.
  Qed.

  Lemma elAt_el A (s : X) m : A .[ m ] = Some s → s ∊ A.
  Proof.
    revert A. induction m; intros []; inversion 1; eauto.
  Qed.

  Lemma el_elAt (s : X) A : s ∊ A → ∃ m, A .[ m ] = Some s.
  Proof.
    intros H; destruct (el_pos H); eexists; eauto using pos_elAt.
  Qed.

Lemma dupfree_elAt (A : list X) n m s : dupfree A → A.[n] = Some s → A.[m] = Some s → n = m.
Proof with try tauto.
  intros H; revert n m; induction A; simpl; intros n m H1 H2.
  - destruct n; inv H1.
  - destruct n, m; inv H...
    + inv H1. simpl in H2. eapply elAt_el in H2...
    + inv H2. simpl in H1. eapply elAt_el in H1...
    + inv H1. inv H2. rewrite IHA with n m...
Qed.

Lemma nth_error_none A n l : nth_error l n = @None A → length l ≤ n.
Proof. revert n;
  induction l; intros n.
  - simpl; omega.
  - simpl. intros. destruct n. inv H. inv H. assert (| l | ≤ n). eauto. omega.
Qed.

End pos.

Arguments pos {_} {_} _ _.
Arguments el_elAt {_} {_} {_} _ _.