193 lines
8.4 KiB
Text
193 lines
8.4 KiB
Text
/-
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Copyright (c) 2015 Microsoft Corporation. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Author: Jeremy Avigad
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Cardinality calculations for finite sets.
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-/
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import .to_set .bigops data.set.function data.nat.power data.nat.bigops
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open nat eq.ops
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namespace finset
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variables {A B : Type}
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variables [deceqA : decidable_eq A] [deceqB : decidable_eq B]
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include deceqA
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theorem card_add_card (s₁ s₂ : finset A) : card s₁ + card s₂ = card (s₁ ∪ s₂) + card (s₁ ∩ s₂) :=
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begin
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induction s₂ with a s₂ ans2 IH,
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show card s₁ + card (∅:finset A) = card (s₁ ∪ ∅) + card (s₁ ∩ ∅),
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by rewrite [union_empty, card_empty, inter_empty],
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show card s₁ + card (insert a s₂) = card (s₁ ∪ (insert a s₂)) + card (s₁ ∩ (insert a s₂)),
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from decidable.by_cases
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(assume as1 : a ∈ s₁,
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assert H : a ∉ s₁ ∩ s₂, from assume H', ans2 (mem_of_mem_inter_right H'),
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begin
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rewrite [card_insert_of_not_mem ans2, union.comm, -insert_union, union.comm],
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rewrite [insert_union, insert_eq_of_mem as1, insert_eq, inter.distrib_left, inter.comm],
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rewrite [singleton_inter_of_mem as1, -insert_eq, card_insert_of_not_mem H, -*add.assoc],
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rewrite IH
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end)
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(assume ans1 : a ∉ s₁,
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assert H : a ∉ s₁ ∪ s₂, from assume H',
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or.elim (mem_or_mem_of_mem_union H') (assume as1, ans1 as1) (assume as2, ans2 as2),
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begin
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rewrite [card_insert_of_not_mem ans2, union.comm, -insert_union, union.comm],
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rewrite [card_insert_of_not_mem H, insert_eq, inter.distrib_left, inter.comm],
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rewrite [singleton_inter_of_not_mem ans1, empty_union, add.right_comm],
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rewrite [-add.assoc, IH]
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end)
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end
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theorem card_union (s₁ s₂ : finset A) : card (s₁ ∪ s₂) = card s₁ + card s₂ - card (s₁ ∩ s₂) :=
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calc
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card (s₁ ∪ s₂) = card (s₁ ∪ s₂) + card (s₁ ∩ s₂) - card (s₁ ∩ s₂) : add_sub_cancel
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... = card s₁ + card s₂ - card (s₁ ∩ s₂) : card_add_card
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theorem card_union_of_disjoint {s₁ s₂ : finset A} (H : s₁ ∩ s₂ = ∅) :
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card (s₁ ∪ s₂) = card s₁ + card s₂ :=
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by rewrite [card_union, H]
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theorem card_le_card_of_subset {s₁ s₂ : finset A} (H : s₁ ⊆ s₂) : card s₁ ≤ card s₂ :=
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have H1 : s₁ ∩ (s₂ \ s₁) = ∅,
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from inter_eq_empty (take x, assume H1 H2, not_mem_of_mem_diff H2 H1),
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calc
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card s₂ = card (s₁ ∪ (s₂ \ s₁)) : union_diff_cancel H
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... = card s₁ + card (s₂ \ s₁) : card_union_of_disjoint H1
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... ≥ card s₁ : le_add_right
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section card_image
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open set
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include deceqB
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theorem card_image_eq_of_inj_on {f : A → B} {s : finset A} (H1 : inj_on f (ts s)) :
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card (image f s) = card s :=
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begin
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induction s with a t H IH,
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{ rewrite [card_empty] },
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{ have H2 : ts t ⊆ ts (insert a t), by rewrite [-subset_eq_to_set_subset]; apply subset_insert,
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have H3 : card (image f t) = card t, from IH (inj_on_of_inj_on_of_subset H1 H2),
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have H4 : f a ∉ image f t,
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proof
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assume H5 : f a ∈ image f t,
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obtain x (H6l : x ∈ t) (H6r : f x = f a), from exists_of_mem_image H5,
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have H7 : x = a, from H1 (mem_insert_of_mem _ H6l) !mem_insert H6r,
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show false, from H (H7 ▸ H6l)
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qed,
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calc
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card (image f (insert a t)) = card (insert (f a) (image f t)) : image_insert
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... = card (image f t) + 1 : card_insert_of_not_mem H4
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... = card t + 1 : H3
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... = card (insert a t) : card_insert_of_not_mem H
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}
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end
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lemma card_le_of_inj_on (a : finset A) (b : finset B)
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(Pex : ∃ f : A → B, set.inj_on f (ts a) ∧ (image f a ⊆ b)):
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card a ≤ card b :=
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obtain f Pinj, from Pex,
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assert Psub : _, from and.right Pinj,
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assert Ple : card (image f a) ≤ card b, from card_le_card_of_subset Psub,
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by rewrite [(card_image_eq_of_inj_on (and.left Pinj))⁻¹]; exact Ple
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theorem card_image_le (f : A → B) (s : finset A) : card (image f s) ≤ card s :=
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finset.induction_on s
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(by rewrite finset.image_empty)
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(take a s',
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assume Ha : a ∉ s',
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assume IH : card (image f s') ≤ card s',
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begin
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rewrite [image_insert, card_insert_of_not_mem Ha],
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apply le.trans !card_insert_le,
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apply add_le_add_right IH
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end)
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theorem inj_on_of_card_image_eq {f : A → B} {s : finset A} :
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card (image f s) = card s → inj_on f (ts s) :=
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finset.induction_on s
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(by intro H; rewrite to_set_empty; apply inj_on_empty)
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(begin
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intro a s' Ha IH,
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rewrite [image_insert, card_insert_of_not_mem Ha, to_set_insert],
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assume H1 : card (insert (f a) (image f s')) = card s' + 1,
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show inj_on f (set.insert a (ts s')), from
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decidable.by_cases
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(assume Hfa : f a ∈ image f s',
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have H2 : card (image f s') = card s' + 1,
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by rewrite [card_insert_of_mem Hfa at H1]; assumption,
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absurd
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(calc
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card (image f s') ≤ card s' : !card_image_le
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... < card s' + 1 : lt_succ_self
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... = card (image f s') : H2)
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!lt.irrefl)
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(assume Hnfa : f a ∉ image f s',
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have H2 : card (image f s') + 1 = card s' + 1,
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by rewrite [card_insert_of_not_mem Hnfa at H1]; assumption,
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have H3 : card (image f s') = card s', from add.cancel_right H2,
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have injf : inj_on f (ts s'), from IH H3,
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show inj_on f (set.insert a (ts s')), from
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take x1 x2,
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assume Hx1 : x1 ∈ set.insert a (ts s'),
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assume Hx2 : x2 ∈ set.insert a (ts s'),
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assume feq : f x1 = f x2,
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or.elim Hx1
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(assume Hx1' : x1 = a,
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or.elim Hx2
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(assume Hx2' : x2 = a, by rewrite [Hx1', Hx2'])
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(assume Hx2' : x2 ∈ ts s',
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have Hfa : f a ∈ image f s',
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by rewrite [-Hx1', feq]; apply mem_image_of_mem f Hx2',
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absurd Hfa Hnfa))
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(assume Hx1' : x1 ∈ ts s',
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or.elim Hx2
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(assume Hx2' : x2 = a,
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have Hfa : f a ∈ image f s',
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by rewrite [-Hx2', -feq]; apply mem_image_of_mem f Hx1',
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absurd Hfa Hnfa)
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(assume Hx2' : x2 ∈ ts s', injf Hx1' Hx2' feq)))
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end)
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end card_image
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theorem Sum_const_eq_card_mul (s : finset A) (n : nat) : (∑ x ∈ s, n) = card s * n :=
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begin
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induction s with a s' H IH,
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rewrite [Sum_empty, card_empty, zero_mul],
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rewrite [Sum_insert_of_not_mem _ H, IH, card_insert_of_not_mem H, add.comm,
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mul.right_distrib, one_mul]
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end
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theorem Sum_one_eq_card (s : finset A) : (∑ x ∈ s, (1 : nat)) = card s :=
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eq.trans !Sum_const_eq_card_mul !mul_one
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section deceqB
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include deceqB
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theorem card_Union_of_disjoint (s : finset A) (f : A → finset B) :
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(∀{a₁ a₂}, a₁ ∈ s → a₂ ∈ s → a₁ ≠ a₂ → f a₁ ∩ f a₂ = ∅) →
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card (⋃ x ∈ s, f x) = ∑ x ∈ s, card (f x) :=
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finset.induction_on s
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(assume H, by rewrite [Union_empty, Sum_empty, card_empty])
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(take a s', assume H : a ∉ s',
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assume IH,
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assume H1 : ∀ {a₁ a₂ : A}, a₁ ∈ insert a s' → a₂ ∈ insert a s' → a₁ ≠ a₂ → f a₁ ∩ f a₂ = ∅,
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have H2 : ∀ a₁ a₂ : A, a₁ ∈ s' → a₂ ∈ s' → a₁ ≠ a₂ → f a₁ ∩ f a₂ = ∅, from
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take a₁ a₂, assume H3 H4 H5,
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H1 (!mem_insert_of_mem H3) (!mem_insert_of_mem H4) H5,
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assert H6 : card (⋃ (x : A) ∈ s', f x) = ∑ (x : A) ∈ s', card (f x), from IH H2,
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have H7 : ∀ x, x ∈ s' → f a ∩ f x = ∅, from
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take x, assume xs',
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have anex : a ≠ x, from assume aex, (eq.subst aex H) xs',
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H1 !mem_insert (!mem_insert_of_mem xs') anex,
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assert H8 : f a ∩ (⋃ (x : A) ∈ s', f x) = ∅, from
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calc
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f a ∩ (⋃ (x : A) ∈ s', f x) = (⋃ (x : A) ∈ s', f a ∩ f x) : inter_Union
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... = (⋃ (x : A) ∈ s', ∅) : Union_ext H7
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... = ∅ : Union_empty',
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by rewrite [Union_insert, Sum_insert_of_not_mem _ H,
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card_union_of_disjoint H8, H6])
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end deceqB
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end finset
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