diff --git a/library/data/finset.lean b/library/data/finset.lean
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+++ b/library/data/finset.lean
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+/-
+Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Module: data.finset
+Author: Leonardo de Moura
+
+Finite sets
+-/
+import data.nat data.list.perm data.subtype algebra.binary algebra.function logic.identities
+open nat quot list subtype binary function
+open [declarations] perm
+
+definition nodup_list (A : Type) := {l : list A | nodup l}
+
+variable {A : Type}
+
+definition to_nodup_list_of_nodup {l : list A} (n : nodup l) : nodup_list A :=
+tag l n
+
+definition to_nodup_list [h : decidable_eq A] (l : list A) : nodup_list A :=
+@to_nodup_list_of_nodup A (erase_dup l) (nodup_erase_dup l)
+
+namespace finset
+
+private definition eqv (l₁ l₂ : nodup_list A) :=
+perm (elt_of l₁) (elt_of l₂)
+
+local infix ~ := eqv
+
+private definition eqv.refl (l : nodup_list A) : l ~ l :=
+!perm.refl
+
+private definition eqv.symm {l₁ l₂ : nodup_list A} : l₁ ~ l₂ → l₂ ~ l₁ :=
+assume p, perm.symm p
+
+private definition eqv.trans {l₁ l₂ l₃ : nodup_list A} : l₁ ~ l₂ → l₂ ~ l₃ → l₁ ~ l₃ :=
+assume p₁ p₂, perm.trans p₁ p₂
+
+definition nodup_list_setoid [instance] (A : Type) : setoid (nodup_list A) :=
+setoid.mk (@eqv A) (mk_equivalence (@eqv A) (@eqv.refl A) (@eqv.symm A) (@eqv.trans A))
+
+definition finset (A : Type) : Type :=
+quot (nodup_list_setoid A)
+
+definition to_finset_of_nodup (l : list A) (n : nodup l) : finset A :=
+⟦to_nodup_list_of_nodup n⟧
+
+definition to_finset [h : decidable_eq A] (l : list A) : finset A :=
+⟦to_nodup_list l⟧
+
+definition has_decidable_eq [instance] [h : decidable_eq A] : decidable_eq (finset A) :=
+λ s₁ s₂, quot.rec_on_subsingleton₂ s₁ s₂
+  (λ l₁ l₂,
+     match decidable_perm (elt_of l₁) (elt_of l₂) with
+     | decidable.inl e := decidable.inl (quot.sound e)
+     | decidable.inr n := decidable.inr (λ e : ⟦l₁⟧ = ⟦l₂⟧, absurd (quot.exact e) n)
+     end)
+
+definition mem (a : A) (s : finset A) : Prop :=
+quot.lift_on s (λ l, a ∈ elt_of l)
+ (λ l₁ l₂ (e : l₁ ~ l₂), propext (iff.intro
+   (λ ainl₁, mem_perm e ainl₁)
+   (λ ainl₂, mem_perm (perm.symm e) ainl₂)))
+
+infix `∈` := mem
+notation a ∉ b := ¬ mem a b
+
+definition mem_of_mem_list {a : A} {l : nodup_list A} : a ∈ elt_of l → a ∈ ⟦l⟧ :=
+λ ainl, ainl
+
+definition mem_list_of_mem {a : A} {l : nodup_list A} : a ∈ ⟦l⟧ → a ∈ elt_of l :=
+λ ainl, ainl
+
+definition decidable_mem [instance] [h : decidable_eq A] : ∀ (a : A) (s : finset A), decidable (a ∈ s) :=
+λ a s, quot.rec_on_subsingleton s
+  (λ l, match list.decidable_mem a (elt_of l) with
+        | decidable.inl p := decidable.inl (mem_of_mem_list p)
+        | decidable.inr n := decidable.inr (λ p, absurd (mem_list_of_mem p) n)
+        end)
+
+theorem mem_to_finset [h : decidable_eq A] {a : A} {l : list A} : a ∈ l → a ∈ to_finset l :=
+λ ainl, mem_erase_dup ainl
+
+theorem mem_to_finset_of_nodub {a : A} {l : list A} (n : nodup l) : a ∈ l → a ∈ to_finset_of_nodup l n :=
+λ ainl, ainl
+
+/- extensionality -/
+theorem ext {s₁ s₂ : finset A} : (∀ a, a ∈ s₁ ↔ a ∈ s₂) → s₁ = s₂ :=
+quot.induction_on₂ s₁ s₂ (λ l₁ l₂ e, quot.sound (perm_ext (has_property l₁) (has_property l₂) e))
+
+/- empty -/
+definition empty : finset A :=
+to_finset_of_nodup [] nodup_nil
+
+notation `∅` := !empty
+
+theorem not_mem_empty (a : A) : a ∉ ∅ :=
+λ aine : a ∈ ∅, aine
+
+/- card -/
+definition card (s : finset A) : nat :=
+quot.lift_on s
+  (λ l, length (elt_of l))
+  (λ l₁ l₂ p, length_eq_length_of_perm p)
+
+theorem card_empty : card (@empty A) = 0 :=
+rfl
+
+/- insert -/
+section insert
+variable [h : decidable_eq A]
+include h
+
+definition insert (a : A) (s : finset A) : finset A :=
+quot.lift_on s
+  (λ l, to_finset_of_nodup (insert a (elt_of l)) (nodup_insert a (has_property l)))
+  (λ (l₁ l₂ : nodup_list A) (p : l₁ ~ l₂), quot.sound (perm_insert a p))
+
+theorem mem_insert (a : A) (s : finset A) : a ∈ insert a s :=
+quot.induction_on s
+ (λ l : nodup_list A, mem_to_finset_of_nodub _ !list.mem_insert)
+
+theorem mem_insert_of_mem {a : A} {s : finset A} (b : A) : a ∈ s → a ∈ insert b s :=
+quot.induction_on s
+ (λ (l : nodup_list A) (ainl : a ∈ ⟦l⟧), mem_to_finset_of_nodub _ (list.mem_insert_of_mem _ ainl))
+
+theorem card_insert_of_mem {a : A} {s : finset A} : a ∈ s → card (insert a s) = card s :=
+quot.induction_on s
+  (λ (l : nodup_list A) (ainl : a ∈ ⟦l⟧), list.length_insert_of_mem ainl)
+
+theorem card_insert_of_not_mem {a : A} {s : finset A} : a ∉ s → card (insert a s) = card s + 1 :=
+quot.induction_on s
+  (λ (l : nodup_list A) (nainl : a ∉ ⟦l⟧), list.length_insert_of_not_mem nainl)
+end insert
+
+/- erase -/
+section erase
+variable [h : decidable_eq A]
+include h
+
+definition erase (a : A) (s : finset A) : finset A :=
+quot.lift_on s
+  (λ l, to_finset_of_nodup (erase a (elt_of l)) (nodup_erase_of_nodup a (has_property l)))
+  (λ (l₁ l₂ : nodup_list A) (p : l₁ ~ l₂), quot.sound (erase_perm_erase_of_perm a p))
+
+theorem mem_erase (a : A) (s : finset A) : a ∉ erase a s :=
+quot.induction_on s
+  (λ l, list.mem_erase_of_nodup _ (has_property l))
+
+theorem card_erase_of_mem {a : A} {s : finset A} : a ∈ s → card (erase a s) = pred (card s) :=
+quot.induction_on s (λ l ainl, list.length_erase_of_mem ainl)
+
+theorem card_erase_of_not_mem {a : A} {s : finset A} : a ∉ s → card (erase a s) = card s :=
+quot.induction_on s (λ l nainl, list.length_erase_of_not_mem nainl)
+end erase
+
+/- union -/
+section union
+variable [h : decidable_eq A]
+include h
+
+definition union (s₁ s₂ : finset A) : finset A :=
+quot.lift_on₂ s₁ s₂
+  (λ l₁ l₂,
+    to_finset_of_nodup (list.union (elt_of l₁) (elt_of l₂))
+                       (nodup_union_of_nodup_of_nodup (has_property l₁) (has_property l₂)))
+  (λ v₁ v₂ w₁ w₂ p₁ p₂, quot.sound (perm_union p₁ p₂))
+
+notation s₁ ∪ s₂ := union s₁ s₂
+
+theorem mem_union_left {a : A} {s₁ : finset A} (s₂ : finset A) : a ∈ s₁ → a ∈ s₁ ∪ s₂ :=
+quot.induction_on₂ s₁ s₂ (λ l₁ l₂ ainl₁, list.mem_union_left _ ainl₁)
+
+theorem mem_union_right {a : A} {s₂ : finset A} (s₁ : finset A) : a ∈ s₂ → a ∈ s₁ ∪ s₂ :=
+quot.induction_on₂ s₁ s₂ (λ l₁ l₂ ainl₂, list.mem_union_right _ ainl₂)
+
+theorem mem_or_mem_of_mem_union {a : A} {s₁ s₂ : finset A} : a ∈ s₁ ∪ s₂ → a ∈ s₁ ∨ a ∈ s₂ :=
+quot.induction_on₂ s₁ s₂ (λ l₁ l₂ ainl₁l₂, list.mem_or_mem_of_mem_union ainl₁l₂)
+
+theorem mem_union_eq (a : A) (s₁ s₂ : finset A) : (a ∈ s₁ ∪ s₂) = (a ∈ s₁ ∨ a ∈ s₂) :=
+propext (iff.intro
+ (λ h, mem_or_mem_of_mem_union h)
+ (λ d, or.elim d
+   (λ i, mem_union_left _ i)
+   (λ i, mem_union_right _ i)))
+
+theorem union.comm (s₁ s₂ : finset A) : s₁ ∪ s₂ = s₂ ∪ s₁ :=
+ext (λ a, by rewrite [*mem_union_eq]; exact or.comm)
+
+theorem union.assoc (s₁ s₂ s₃ : finset A) : (s₁ ∪ s₂) ∪ s₃ = s₁ ∪ (s₂ ∪ s₃) :=
+ext (λ a, by rewrite [*mem_union_eq]; exact or.assoc)
+
+theorem union_self (s : finset A) : s ∪ s = s :=
+ext (λ a, iff.intro
+  (λ ain, or.elim (mem_or_mem_of_mem_union ain) (λ i, i) (λ i, i))
+  (λ i, mem_union_left _ i))
+
+theorem union_empty (s : finset A) : s ∪ ∅ = s :=
+ext (λ a, iff.intro
+  (λ ain : a ∈ s ∪ ∅, or.elim (mem_or_mem_of_mem_union ain) (λ i, i) (λ i, absurd i !not_mem_empty))
+  (λ i   : a ∈ s, mem_union_left _ i))
+
+theorem empty_union (s : finset A) : ∅ ∪ s = s :=
+calc ∅ ∪ s = s ∪ ∅ : union.comm
+       ... = s     : union_empty
+end union
+
+/- acc -/
+section acc
+variable {B : Type}
+definition acc (f : B → A → B) (rcomm : ∀ b a₁ a₂, f (f b a₁) a₂ = f (f b a₂) a₁) (b : B) (s : finset A) : B :=
+quot.lift_on s (λ l : nodup_list A, list.foldl f b (elt_of l))
+  (λ l₁ l₂ p, foldl_eq_of_perm rcomm p b)
+
+definition bigsum (s : finset A) (f : A → nat) : nat :=
+acc (compose_right nat.add f) (right_commutative_compose_right nat.add f nat.add.right_comm) 0 s
+
+definition bigprod (s : finset A) (f : A → nat) : nat :=
+acc (compose_right nat.mul f) (right_commutative_compose_right nat.mul f nat.mul.right_comm) 1 s
+
+definition bigand (s : finset A) (p : A → Prop) : Prop :=
+acc (compose_right and p) (right_commutative_compose_right and p (λ a b c, propext (and.right_comm a b c))) true s
+
+definition bigor (s : finset A) (p : A → Prop) : Prop :=
+acc (compose_right or p) (right_commutative_compose_right or p (λ a b c, propext (or.right_comm a b c))) false s
+end acc
+end finset