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Module mathcomp.classical.fsbigop

From mathcomp Require Import all_ssreflect ssralg ssrnum ssrint interval finmap.
From mathcomp Require Import mathcomp_extra boolp classical_sets functions.
From mathcomp Require Import cardinality.


Reserved Notation "\big [ op / idx ]_ ( i '\in' A ) F"
  (at level 36, F at level 36, op, idx at level 10, i, A at level 50,
           format "'[' \big [ op / idx ]_ ( i '\in' A ) '/ ' F ']'").

Reserved Notation "\sum_ ( i '\in' A ) F"
  (at level 41, F at level 41, i, A at level 50,
    format "'[' \sum_ ( i '\in' A ) '/ ' F ']'").

Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Import Order.TTheory GRing.Theory Num.Def Num.Theory.

Local Open Scope classical_set_scope.
Local Open Scope ring_scope.

Notation "\big [ op / idx ]_ ( i '\in' A ) F" :=
  (\big[op/idx]_(i <- fset_set (A `&` ((fun i => F) @^-1` [set~ idx]))) F)
    (only parsing) : big_scope.

Lemma finite_index_key : unit. Proof. exact: tt.Definition finite_support {I : choiceType} {T : Type} (idx : T) (D : set I)
    (F : I -> T) : seq I :=
  locked_with finite_index_key (fset_set (D `&` F @^-1` [set~ idx] : set I)).
Notation "\big [ op / idx ]_ ( i '\in' D ) F" :=
    (\big[op/idx]_(i <- finite_support idx D (fun i => F)) F)
  : big_scope.

Lemma in_finite_support (T : Type) (J : choiceType) (i : T) (P : set J)
    (F : J -> T) : finite_set (P `&` F @^-1` [set~ i]) ->
  finite_support i P F =i P `&` F @^-1` [set~ i].

Proof.

by move=> finF j; rewrite /finite_support unlock in_fset_set. Qed.

Lemma finite_support_uniq (T : Type) (J : choiceType) (i : T) (P : set J)
    (F : J -> T) : uniq (finite_support i P F).

Proof.

by rewrite /finite_support unlock; exact: fset_uniq. Qed.

#[global] Hint Resolve finite_support_uniq : core.

Lemma no_finite_support (T : Type) (J : choiceType) (i : T) (P : set J)
    (F : J -> T) : infinite_set (P `&` F @^-1` [set~ i]) ->
  finite_support i P F = [::].

Proof.

move=> infinF; rewrite /finite_support unlock.
by rewrite /fset_set/=; case: pselect => //.
Qed.

Lemma eq_finite_support {I : choiceType} {T : Type} (idx : T) (D : set I)
    (F G : I -> T) : {in D, F =1 G} ->
  finite_support idx D F = finite_support idx D G.

Proof.

by move=> eqFG; rewrite /finite_support !unlock// (eq_preimage _ eqFG).
Qed.

Variant finite_support_spec R (T : choiceType)
  (P : set T) (F : T -> R) (idx : R) : seq T -> Type :=
| NoFiniteSupport of infinite_set (P `&` F @^-1` [set~ idx]) :
    finite_support_spec P F idx [::]
| FiniteSupport (X : {fset T}) of [set` X] `<=` P
   & (forall i, P i -> i \notin X -> F i = idx)
   & [set` X] = (P `&` F @^-1` [set~ idx]) :
    finite_support_spec P F idx X.

Lemma finite_supportP R (T : choiceType) (P : set T) (F : T -> R) (idx : R) :
  finite_support_spec P F idx (finite_support idx P F).

Proof.

rewrite /finite_support unlock/= /fset_set.
case: pselect=> // Xfin; last by constructor.
case: cid => //= X eqX; constructor; rewrite -?eqX//.
move=> i Pi NXi /=; have : (P `\` [set` X]) i by split=> //=; apply/negP.
by rewrite -eqX /= => -[_]; apply: contra_notP.
Qed.

Notation "\sum_ ( i '\in' A ) F" := (\big[+%R/0%R]_(i \in A) F) : ring_scope.

Lemma eq_fsbigl (R : Type) (idx : R) (op : R -> R -> R)
    (T : choiceType) (f : T -> R) (P Q : set T) :
  P = Q -> \big[op/idx]_(x \in P) f x = \big[op/idx]_(x \in Q) f x.

Proof.

by move=> ->. Qed.

Lemma eq_fsbigr (R : Type) (idx : R) (op : Monoid.com_law idx)
    (T : choiceType) (f g : T -> R) (P : set T) :
  {in P, f =1 g} -> (\big[op/idx]_(x \in P) f x = \big[op/idx]_(x \in P) g x).

Proof.

move=> fg; rewrite (eq_finite_support _ fg); apply: eq_big_seq => x.
by case: finite_supportP => //= X XP _ gidx xX; rewrite fg // ?inE; apply/XP.
Qed.

Arguments eq_fsbigr {R idx op T f} g.

Lemma fsbigTE (R : Type) (idx : R) (op : Monoid.com_law idx) (T : choiceType)
    (A : {fset T}) (f : T -> R) :
    (forall i, i \notin A -> f i = idx) ->
  \big[op/idx]_(i \in [set: T]) f i = \big[op/idx]_(i <- A) f i.

Proof.

elim/Peq: R => R in idx op f *.
move=> Af; have Afin : finite_set (f @^-1` [set~ idx]).
  by apply: (finite_subfset A) => x; apply: contra_notT => /Af.
rewrite [in RHS](big_fsetID _ [pred x | f x == idx])/=.
rewrite [X in _ = op X _]big_fset [X in _ = op X _]big1 ?Monoid.simpm//; last first.
  by move=> i /= /eqP.
apply eq_fbigl => r.
rewrite in_finite_support// ?setTI// /preimage/=; apply/idP/idP => /=.
  rewrite !inE/=; apply: contra_notP => /negP.
  by rewrite negb_and negbK => /orP[|/eqP//]; exact: Af.
by rewrite !inE/= => /andP[_ /eqP].
Qed.

Arguments fsbigTE {R idx op T} A f.

Lemma fsbig_mkcond (R : Type) (idx : R) (op : Monoid.com_law idx)
    (T : choiceType) (A : set T) (f : T -> R) :
  \big[op/idx]_(i \in A) f i =
  \big[op/idx]_(i \in [set: T]) patch (fun=> idx) A f i.

Proof.

elim/Peq: R => R in idx op f *.
rewrite -big_mkcond/= -[in RHS]big_filter; apply: perm_big.
rewrite uniq_perm ?filter_uniq//= => i; rewrite mem_filter.
set g := fun i => if i \in A then f i else idx.
have gAf : setT `&` g @^-1` [set~ idx] = (A `&` f @^-1` [set~ idx]).
  rewrite setTI; apply/predeqP => x; split; rewrite /preimage/g/=.
    by case: ifPn; rewrite (inE, notin_setE).
  by case: ifPn; rewrite (inE, notin_setE) => ? [].
case: finite_supportP => //.
  rewrite -gAf; case: finite_supportP=> //=; first by rewrite ?inE andbF.
  by move=> X _ gidx <-//.
move=> X XA fidx XE; case: finite_supportP; rewrite gAf -?XE//=.
move=> Y _ gidx /predeqP/=/(_ _)/propext YX.
by apply/idP/andP => [|[]]; rewrite YX// inE => Xi; split=> //; apply: XA.
Qed.

Lemma fsbig_mkcondr (R : Type) (idx : R) (op : Monoid.com_law idx)
    (T : choiceType) (I J : set T) (a : T -> R) :
  \big[op/idx]_(i \in I `&` J) a i =
  \big[op/idx]_(i \in I) if i \in J then a i else idx.

Proof.

rewrite fsbig_mkcond [RHS]fsbig_mkcond.
by under eq_fsbigr do rewrite patch_setI.
Qed.

Lemma fsbig_mkcondl (R : Type) (idx : R) (op : Monoid.com_law idx)
    (T : choiceType) (I J : set T) (a : T -> R) :
  \big[op/idx]_(i \in I `&` J) a i =
  \big[op/idx]_(i \in J) if i \in I then a i else idx.

Proof.

rewrite fsbig_mkcond [RHS]fsbig_mkcond setIC.
by under eq_fsbigr do rewrite patch_setI.
Qed.

Lemma bigfs (R : Type) (idx : R) (op : Monoid.com_law idx) (T : choiceType)
    (r : seq T) (P : {pred T}) (f : T -> R) : uniq r ->
    (forall i, P i -> i \notin r -> f i = idx) ->
  \big[op/idx]_(i <- r | P i) f i = \big[op/idx]_(i \in [set` P]) f i.

Proof.

move=> r_uniq fidx; rewrite fsbig_mkcond.
rewrite (fsbigTE [fset x | x in r]%fset); last first.
  by move=> i; rewrite inE/= /patch mem_setE; case: ifP=> // + /fidx->.
rewrite -big_mkcond; under [RHS]eq_bigl do rewrite mem_setE.
by apply: perm_big; rewrite uniq_perm// => i; rewrite !inE.
Qed.

Lemma fsbigE (R : Type) (idx : R) (op : Monoid.com_law idx) (T : choiceType)
    (A : set T) (r : seq T) (f : T -> R) :
    uniq r ->
    [set` r] `<=` A ->
    (forall i, A i -> i \notin r -> f i = idx) ->
  \big[op/idx]_(i \in A) f i = \big[op/idx]_(i <- r | i \in A) f i.

Proof.

move=> r_uniq rQ fidx; rewrite [RHS]bigfs ?set_mem_set//=.
by move=> i; rewrite inE; apply: fidx.
Qed.

Arguments fsbigE {R idx op T A}.

Lemma fsbig_seq (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I : choiceType) (r : seq I) (F : I -> R) :
  uniq r ->
  \big[op/idx]_(a <- r) F a = \big[op/idx]_(a \in [set` r]) F a.

Proof.

move=> ur; rewrite (fsbigE r)//=; last by move=> + ->.
by rewrite mem_setE big_seq_cond big_mkcondr.
Qed.

Lemma fsbig1 (R : Type) (idx : R) (op : Monoid.law idx) (I : choiceType)
    (P : set I) (F : I -> R) :
  (forall i, P i -> F i = idx) -> \big[op/idx]_(i \in P) F i = idx.

Proof.

move=> PF0; rewrite big1_seq// => i/=; case: finite_supportP=> //=.
by move=> X XP _ _ Xi; rewrite PF0//; apply/XP.
Qed.

Lemma fsbig_dflt (R : Type) (idx : R) (op : Monoid.law idx) (I : choiceType)
    (P : set I) (F : I -> R) :
  infinite_set (P `&` F @^-1` [set~ idx])-> \big[op/idx]_(i \in P) F i = idx.

Proof.

by case: finite_supportP; rewrite ?big_nil// => X _ _ <-. Qed.

Lemma fsbig_widen (T : choiceType) [R : Type] [idx : R]
    (op : Monoid.com_law idx) (P D : set T) (f : T -> R) :
    P `<=` D ->
    D `\` P `<=` f @^-1` [set idx] ->
  \big[op/idx]_(i \in P) f i = \big[op/idx]_(i \in D) f i.

Proof.

move=> PD DPf; rewrite fsbig_mkcond [RHS]fsbig_mkcond.
apply: eq_fsbigr => x _; rewrite /patch; case: ifPn; rewrite (inE, notin_setE).
  by move=> Px; rewrite ifT// inE; apply: PD.
by move=> Px; case: ifP => //; rewrite inE => Dx; rewrite DPf.
Qed.

Arguments fsbig_widen {T R idx op} P D f.

Lemma fsbig_supp (T : choiceType) [R : Type] [idx : R]
    (op : Monoid.com_law idx) (P : set T) (f : T -> R) :
  \big[op/idx]_(i \in P) f i = \big[op/idx]_(i \in P `&` f @^-1` [set~ idx]) f i.

Proof.

by apply/esym/fsbig_widen => // x [Px /not_andP[]//=]; rewrite notK. Qed.

Lemma fsbig_fwiden (T : choiceType) [R : eqType] [idx : R]
    (op : Monoid.com_law idx)
    (r : seq T) (P : set T) (f : T -> R) :
  P `<=` [set` r] ->
  uniq r ->
  [set i | i \in r] `\` P `<=` f @^-1` [set idx] ->
  \big[op/idx]_(i \in P) f i = \big[op/idx]_(i <- r) f i.

Proof.

by move=> *; rewrite (fsbig_widen _ [set` r])// [RHS]fsbig_seq. Qed.

Arguments fsbig_fwiden {T R idx op} r P f.

Lemma fsbig_set0 (R : Type) (idx : R) (op : Monoid.com_law idx) (T : choiceType)
  (F : T -> R) : \big[op/idx]_(x \in set0) F x = idx.

Proof.

by rewrite (fsbigE [::])// big_nil. Qed.

Lemma fsbig_set1 (R : Type) (idx : R) (op : Monoid.com_law idx) (T : choiceType) x
  (F : T -> R) : \big[op/idx]_(y \in [set x]) F y = F x.

Proof.

rewrite (fsbigE [:: x])//= ?big_cons ?big_nil ?ifT ?inE ?Monoid.simpm//.
  by move=> y /=; rewrite inE => /eqP.
by move=> i ->; rewrite inE eqxx.
Qed.

Lemma __deprecated__full_fsbigID (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I : choiceType) (B : set I) (A : set I) (F : I -> R) :
  finite_set (A `&` F @^-1` [set~ idx]) ->
  \big[op/idx]_(i \in A) F i = op (\big[op/idx]_(i \in A `&` B) F i)
                                  (\big[op/idx]_(i \in A `&` ~` B) F i).

Proof.

move=> finF.
have fsbig_setI C : \big[op/idx]_(i <-
      [fset x | x in fset_set (A `&` F @^-1` [set~ idx]) & x \in C]%fset) F i =
    \big[op/idx]_(i \in A `&` C) F i.
  apply: eq_fbigl => i /=; apply/idP/idP.
    rewrite !inE/= => /andP[+ Bi]; rewrite in_fset_set// inE => -[Ai Fi].
    rewrite unlock in_fset_set ?inE// setIAC; first by rewrite inE in Bi.
    exact/finite_setIl.
  rewrite unlock in_fset_set; last by rewrite setIAC; exact/finite_setIl.
  by rewrite inE => -[[Ai Bi] Fi0]; rewrite !inE/= in_fset_set// !mem_set.
rewrite (big_fsetID _ [pred i | i \in B])/= [locked_with _ _]unlock.
rewrite fsbig_setI; congr (op _ _); rewrite -fsbig_setI.
by apply eq_fbigl => i; rewrite !inE in_setC.
Qed.

Arguments __deprecated__full_fsbigID {R idx op I} B.

Lemma fsbigID (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I : choiceType) (B : set I) (A : set I) (F : I -> R) :
    finite_set A ->
  \big[op/idx]_(i \in A) F i = op (\big[op/idx]_(i \in A `&` B) F i)
                                  (\big[op/idx]_(i \in A `&` ~` B) F i).

Proof.

by move=> Afin; apply: __deprecated__full_fsbigID; apply: finite_setIl. Qed.

Arguments fsbigID {R idx op I} B.

#[deprecated(note="Use fsbigID instead")]
Notation full_fsbigID := __deprecated__full_fsbigID (only parsing).

Lemma fsbigU (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I : choiceType) (A B : set I) (F : I -> R) :
    finite_set A -> finite_set B -> A `&` B `<=` F @^-1` [set idx] ->
  \big[op/idx]_(i \in A `|` B) F i =
     op (\big[op/idx]_(i \in A) F i) (\big[op/idx]_(i \in B) F i).

Proof.

move=> Afin Bfin AB0; rewrite (fsbigID A) ?finite_setU; last by split.
rewrite setUK -setDE; congr (op _ _); rewrite setDE setIUl setICr set0U.
by apply: fsbig_widen => //; rewrite -setDE setDD setIC.
Qed.

Arguments fsbigU {R idx op I} [A B F].

Lemma fsbigU0 (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I : choiceType) (A B : set I) (F : I -> R) :
    finite_set A -> finite_set B -> A `&` B `<=` set0 ->
  \big[op/idx]_(i \in A `|` B) F i =
     op (\big[op/idx]_(i \in A) F i) (\big[op/idx]_(i \in B) F i).

Proof.

by move=> Af Bf AB0; rewrite fsbigU// => x /AB0. Qed.

Lemma fsbigD1 (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I : choiceType) (i : I) (A : set I) (F : I -> R) :
     finite_set A -> A i ->
  \big[op/idx]_(j \in A) F j = op (F i) (\big[op/idx]_(j \in A `\ i) F j).

Proof.

by move=> *; rewrite (fsbigID [set i]) ?setI1 ?ifT ?inE ?fsbig_set1. Qed.

Arguments fsbigD1 {R idx op I} i A F.

Lemma full_fsbig_distrr (R : Type) (zero : R) (times : Monoid.mul_law zero)
    (plus : Monoid.add_law zero times) (I : choiceType) (a : R) (P : set I)
    (F : I -> R) :
  finite_set (P `&` F @^-1` [set~ zero]) ->
  times a (\big[plus/zero]_(i \in P) F i) =
  \big[plus/zero]_(i \in P) times a (F i).

Proof.

move=> finF; elim/Peq : R => R in zero times plus a F finF *.
have [->|a0] := eqVneq a zero.
  by rewrite Monoid.mul0m fsbig1//; move=> i _; rewrite Monoid.mul0m.
rewrite big_distrr [RHS](full_fsbigID (F @^-1` [set zero])); last first.
  apply: sub_finite_set finF => x /= [Px aFN0].
  by split=> //; apply: contra_not aFN0 => ->; rewrite Monoid.simpm.
set b0 := bigop _ _ _.
set b1 := bigop _ _ _.
set b2 := bigop _ _ _.
rewrite (_ : b1 = zero) ?Monoid.simpm; last first.
  by rewrite /b1 fsbig1// => i [_ ->]; rewrite Monoid.simpm.
apply/esym/fsbig_fwiden => //.
  by move=> x [Px Fx0]; rewrite /= in_finite_support// inE.
move=> i []; rewrite /preimage/= in_finite_support //.
by rewrite !inE => -[Pi]; rewrite /preimage/= => Fi0; tauto.
Qed.

Lemma fsbig_distrr (R : Type) (zero : R) (times : Monoid.mul_law zero)
    (plus : Monoid.add_law zero times) (I : choiceType) (a : R) (P : set I)
    (F : I -> R) :
  finite_set P ->
  times a (\big[plus/zero]_(i \in P) F i) =
  \big[plus/zero]_(i \in P) times a (F i).

Proof.

by move=> Pf; apply: full_fsbig_distrr; apply: finite_setIl. Qed.

Lemma mulr_fsumr (R : idomainType) (I : choiceType) a (P : set I) (F : I -> R) :
   a * (\sum_(i \in P) F i) = \sum_(i \in P) a * F i.

Proof.

have [->|aN0] := eqVneq a 0; first by rewrite mul0r big1// => i; rewrite mul0r.
case: (pselect (finite_set (P `&` F @^-1` [set~ 0]))) => PFfin.
  exact: full_fsbig_distrr.
rewrite !fsbig_dflt ?mulr0//; apply: contra_not PFfin; apply: sub_finite_set.
by move=> x [Px /eqP Fx0]; split=> //=; apply/eqP; rewrite mulf_neq0.
Qed.

Lemma mulr_fsuml (R : idomainType) (I : choiceType) a (P : set I) (F : I -> R) :
   (\sum_(i \in P) F i) * a = \sum_(i \in P) (F i * a).

Proof.

by rewrite mulrC mulr_fsumr; under eq_fsbigr do rewrite mulrC. Qed.

Lemma fsbig_ord R (idx : R) (op : Monoid.com_law idx) n (F : nat -> R) :
  \big[op/idx]_(a < n) F a = \big[op/idx]_(a \in `I_n) F a.

Proof.

rewrite -(big_mkord xpredT) [LHS]fsbig_seq ?iota_uniq//.
by apply: eq_fsbigl; rewrite -Iiota /index_iota subn0.
Qed.

Lemma fsbig_finite (R : Type) (idx : R) (op : Monoid.com_law idx) (T : choiceType)
    (D : set T) (F : T -> R) : finite_set D ->
  \big[op/idx]_(x \in D) F x = \big[op/idx]_(x <- fset_set D) F x.

Proof.

elim/Peq: R => R in idx op F * => Dfin.
by apply: fsbig_fwiden; rewrite ?fset_setK// setDv.
Qed.

Section fsbig2.
Variables (R : Type) (idx : R) (op : Monoid.com_law idx).

Lemma reindex_fsbig {I J : choiceType} (h : I -> J) P Q
    (F : J -> R) : set_bij P Q h ->
  \big[op/idx]_(j \in Q) F j = \big[op/idx]_(i \in P) F (h i).

Proof.

elim/choicePpointed: I => I in h P *.
  rewrite !emptyE => /Pbij[{}h ->].
  by rewrite -[in LHS](image_eq h) image_set0 !fsbig_set0.
elim/choicePpointed: J => J in F h Q *; first by have := no (h point).
move=> /(@pPbij _ _ _)[{}h ->].
pose A := P `&` (F \o h) @^-1` [set~ idx].
pose B := Q `&` F @^-1` [set~ idx].
have /(@pPbij _ _ _)[g gh] : set_bij A B h.
  apply: splitbij_sub; rewrite /A /B /preimage //=.
    by move=> x [Px Fhx]; split=> //; apply: funS.
  by move=> x [Qx Fx]; split; rewrite ?invK ?inE//; apply: funS.
case: finite_supportP; rewrite ?big_nil//=.
  case: finite_supportP; rewrite ?big_nil//=.
  move=> X XP _ XE []; rewrite -/B -(image_eq g) /A.
  by apply: finite_image; rewrite -XE.
move=> Y YQ Fidx YE; case: finite_supportP.
  move=> []; rewrite -/A -(image_eq [bij of g^-1%FUN]).
  by apply: finite_image; rewrite /B -YE.
move=> X XP Fhidx XE; suff -> : Y = (h @` X)%fset.
  by rewrite big_imfset// => ? ? ? ? /inj; apply; rewrite inE; apply: XP.
have BY j : (B j) = (j \in Y) by rewrite -[RHS]/([set` Y] j) YE.
have AX i : (A i) = (i \in X) by rewrite -[RHS]/([set` X] i) XE.
rewrite gh; apply/fsetP=> j; apply/idP/imfsetP => [Yj | [i iX ->]]; last first.
  by rewrite -BY; apply: funS; rewrite AX.
by exists (g^-1%FUN j); rewrite ?invK ?inE ?BY// -AX; apply: funS; rewrite BY.
Qed.

Lemma fsbig_image {I J : choiceType} P (h : I -> J) (F : J -> R) : set_inj P h ->
  \big[op/idx]_(j \in h @` P) F j = \big[op/idx]_(i \in P) F (h i).

Proof.

by move=> /inj_bij; apply: reindex_fsbig. Qed.

Lemma __deprecated__reindex_inside {I J : choiceType} P Q (h : I -> J) (F : J -> R) :
  bijective h -> P `<=` h @` Q -> Q `<=` h @^-1` P ->
  \big[op/idx]_(j \in P) F j = \big[op/idx]_(i \in Q) F (h i).

Proof.

move=> hbij PQ QP; apply: reindex_fsbig; split=> //.
by move=> x y _ _ /(bij_inj hbij).
Qed.

Lemma reindex_fsbigT {I J : choiceType} (h : I -> J) (F : J -> R) :
  bijective h ->
  \big[op/idx]_(j \in [set: J]) F j = \big[op/idx]_(i \in [set: I]) F (h i).

Proof.

by rewrite -setTT_bijective => -[? ? ?]; apply: reindex_fsbig. Qed.

End fsbig2.
Arguments reindex_fsbig {R idx op I J} _ _ _.
Arguments fsbig_image {R idx op I J} _ _.
Arguments __deprecated__reindex_inside {R idx op I J} _ _.
Arguments reindex_fsbigT {R idx op I J} _ _.
#[deprecated(note="use reindex_fsbig, fsbig_image or reindex_fsbigT instead")]
Notation reindex_inside := __deprecated__reindex_inside (only parsing).
#[deprecated(note="use reindex_fsbigT instead")]
Notation reindex_inside_setT := reindex_fsbigT (only parsing).

Lemma fsbigN1 (R : eqType) (idx : R) (op : Monoid.com_law idx)
    (T1 T2 : choiceType) (Q : set T2) (f : T1 -> T2 -> R) (x : T1) :
  \big[op/idx]_(y \in Q) f x y != idx -> exists2 y, Q y & f x y != idx.

Proof.

apply: contra_neqP => /forall2NP Qf; apply/fsbig1 => y Qy.
by case: (Qf y) => // /negP/negPn/eqP->.
Qed.

Lemma fsbig_split (T : choiceType) (R : eqType) (idx : R)
    (op : Monoid.com_law idx) (P : set T) (f g : T -> R) : finite_set P ->
  \big[op/idx]_(x \in P) op (f x) (g x) =
  op (\big[op/idx]_(x \in P) f x) (\big[op/idx]_(x \in P) g x).

Proof.

by move=> Pfin; rewrite !fsbig_finite// big_split. Qed.

Lemma fsumr_ge0 (R : numDomainType) (I : choiceType) (P : set I) (F : I -> R) :
  (forall i, P i -> 0 <= F i) -> 0 <= \sum_(i \in P) F i.

Proof.

move=> PF; case: finite_supportP; rewrite ?big_nil// => X XP F0 _.
by rewrite big_seq_cond big_mkcondr sumr_ge0// => i /XP/PF.
Qed.

Lemma fsumr_le0 (R : numDomainType) (I : choiceType) (P : set I) (F : I -> R) :
  (forall i, P i -> F i <= 0) -> \sum_(i \in P) F i <= 0.

Proof.

move=> PF; case: finite_supportP; rewrite ?big_nil// => X XP F0 _.
by rewrite big_seq_cond big_mkcondr sumr_le0// => i /XP/PF.
Qed.

Lemma fsumr_gt0 (R : realDomainType) (I : choiceType) (r : seq I) (P : set I)
    (F : I -> R) :
  0 < \sum_(i \in P) F i -> exists2 i, P i & 0 < F i.

Proof.

apply: contraPP => /forall2NP xNPF; rewrite le_gtF// fsumr_le0// => i Pi.
by case: (xNPF i) => // /negP; case: ltP.
Qed.

Lemma fsumr_lt0 (R : realDomainType) (I : choiceType) (P : set I)
    (F : I -> R) :
  \sum_(i \in P) F i < 0 -> exists2 i, P i & F i < 0.

Proof.

apply: contraPP => /forall2NP xNPF; rewrite le_gtF// fsumr_ge0// => i Pi.
by case: (xNPF i) => // /negP; case: ltP.
Qed.

Lemma pfsumr_eq0 (R : realDomainType) (I : choiceType) (P : set I)
    (F : I -> R) :
  finite_set P ->
  (forall i, P i -> 0 <= F i) ->
  \sum_(i \in P) F i = 0 -> forall i, P i -> F i = 0.

Proof.

move=> Pfin F0 /eqP; apply: contraTP => /existsPNP[i Pi /eqP Fi0].
rewrite (fsbigD1 i)//= paddr_eq0 ?F0 ?negb_and ?Fi0//.
by rewrite fsumr_ge0// => j [/F0->].
Qed.

Lemma fsbig_setU {T} {I : choiceType} (A : set I) (F : I -> set T) :
  finite_set A ->
  \big[setU/set0]_(i \in A) F i = \bigcup_(i in A) F i.

Proof.

by move=> Afin; rewrite fsbig_finite// bigsetU_fset_set. Qed.

Lemma fsbig_setU_set1 {T : choiceType} (A : set T) :
  finite_set A -> \big[setU/set0]_(x \in A) [set x] = A.

Proof.

move=> fA; rewrite fsbig_setU//.
by apply/seteqP; split=> [t [x Ax ->]//|t At]; exists t.
Qed.

Lemma pair_fsbig (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I J : choiceType) (P : set I) (Q : set J) (F : I -> J -> R) :
  finite_set P -> finite_set Q ->
  \big[op/idx]_(i \in P) \big[op/idx]_(j \in Q) F i j
  = \big[op/idx]_(p \in P `*` Q) F p.1 p.2.

Proof.

move=> Pfin Qfin; have PQfin : finite_set (P `*` Q) by exact: finite_setX.
rewrite !fsbig_finite//=; under eq_bigr do rewrite fsbig_finite//=.
rewrite pair_big_dep_cond/= fset_setX//.
apply: eq_fbigl => -[i j] //=; apply/imfset2P/idP; rewrite inE //=.
  by move=> [x + [y + [-> ->]]]; rewrite 4!inE/= !andbT/= => -> ->.
move=> /andP[Pi Qi]; exists i; rewrite 2?inE ?andbT//.
by exists j; rewrite 2?inE ?andbT.
Qed.

Lemma exchange_fsbig (R : Type) (idx : R) (op : Monoid.com_law idx)
    (I J : choiceType) (P : set I) (Q : set J) (F : I -> J -> R) :
  finite_set P -> finite_set Q ->
  \big[op/idx]_(i \in P) \big[op/idx]_(j \in Q) F i j
  = \big[op/idx]_(j \in Q) \big[op/idx]_(i \in P) F i j.

Proof.

move=> Pfin Qfin; rewrite 2?pair_fsbig//; pose swap (x : I * J) := (x.2, x.1).
apply/esym/(reindex_fsbig swap).
split=> [? [? ?]//|[? ?] [? ?] /set_mem[? ?] /set_mem[? ?] [-> ->]//|].
by move=> [i j] [? ?]; exists (j, i).
Qed.