-- Mathlib/Combinatorics/SetFamily/Shatter.lean
/-- Pajor's variant of the **Sauer-Shelah lemma**. -/
lemma card_le_card_shatterer (𝒜 : Finset (Finset α)) : #𝒜 ≤ #𝒜.shatterer

Proof state:
case refine_3.refine_1.intro.intro
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s : Finset α
hs : (memberSubfamily a 𝒜).Shatters s
u : Finset α
hu : insert a u ∈ 𝒜 ∧ a ∉ u
ht : s ∩ u ⊆ s
⊢ ∃ u_1 ∈ 𝒜, s ∩ u_1 = s ∩ u

Tactic:
refine ⟨insert a u, hu.1, inter_insert_of_not_mem fun ha ↦ ?_⟩

Result:
case refine_3.refine_1.intro.intro
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s : Finset α
hs : (memberSubfamily a 𝒜).Shatters s
u : Finset α
hu : insert a u ∈ 𝒜 ∧ a ∉ u
ht : s ∩ u ⊆ s
ha : a ∈ s
⊢ False

Proof state:
case refine_3
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
⊢ #𝒜 ≤ #𝒜.shatterer

Tactic:
set ℬ : Finset (Finset α) :=
  ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer).image (insert a)

Result:
case refine_3
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
⊢ #𝒜 ≤ #𝒜.shatterer

Proof state:
case pos
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s t : Finset α
ht : t.erase a ⊆ s
hs : (memberSubfamily a 𝒜).Shatters s ∧ (nonMemberSubfamily a 𝒜).Shatters s
ha : a ∈ t
⊢ ∃ u ∈ 𝒜, insert a s ∩ u = t

Tactic:
obtain ⟨u, hu, hsu⟩ := hs.1 ht

Result:
case pos.intro.intro
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s t : Finset α
ht : t.erase a ⊆ s
hs : (memberSubfamily a 𝒜).Shatters s ∧ (nonMemberSubfamily a 𝒜).Shatters s
ha : a ∈ t
u : Finset α
hu : u ∈ memberSubfamily a 𝒜
hsu : s ∩ u = t.erase a
⊢ ∃ u ∈ 𝒜, insert a s ∩ u = t

Proof state:
case neg.intro.intro
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s t : Finset α
ht : t.erase a ⊆ s
hs : (memberSubfamily a 𝒜).Shatters s ∧ (nonMemberSubfamily a 𝒜).Shatters s
ha : a ∉ t
u : Finset α
hu : u ∈ 𝒜 ∧ a ∉ u
hsu : s ∩ u = t.erase a
⊢ insert a s ∩ u = t

Tactic:
rwa [insert_inter_of_not_mem hu.2, hsu, erase_eq_self]

Result:
no goals

Proof state:
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
⊢ #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)

Tactic:
refine card_image_of_injOn <| insert_erase_invOn.2.injOn.mono ?_

Result:
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
⊢ ↑((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer) ⊆ {s | a ∉ s}

Proof state:
α : Type u_1
inst✝ : DecidableEq α
𝒜 : Finset (Finset α)
⊢ #𝒜 ≤ #𝒜.shatterer

Tactic:
refine memberFamily_induction_on 𝒜 ?_ ?_ ?_

Result:
case refine_1
α : Type u_1
inst✝ : DecidableEq α
𝒜 : Finset (Finset α)
⊢ #∅ ≤ #∅.shatterer

case refine_2
α : Type u_1
inst✝ : DecidableEq α
𝒜 : Finset (Finset α)
⊢ #{∅} ≤ #{∅}.shatterer

case refine_3
α : Type u_1
inst✝ : DecidableEq α
𝒜 : Finset (Finset α)
⊢ ∀ (a : α) ⦃𝒜 : Finset (Finset α)⦄,
    #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer →
      #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer → #𝒜 ≤ #𝒜.shatterer

Proof state:
case neg
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s t : Finset α
ht : t.erase a ⊆ s
hs : (memberSubfamily a 𝒜).Shatters s ∧ (nonMemberSubfamily a 𝒜).Shatters s
ha : a ∉ t
⊢ ∃ u ∈ 𝒜, insert a s ∩ u = t

Tactic:
obtain ⟨u, hu, hsu⟩ := hs.2 ht

Result:
case neg.intro.intro
α : Type u_1
inst✝ : DecidableEq α
𝒜✝ : Finset (Finset α)
a : α
𝒜 : Finset (Finset α)
ih₀ : #(nonMemberSubfamily a 𝒜) ≤ #(nonMemberSubfamily a 𝒜).shatterer
ih₁ : #(memberSubfamily a 𝒜) ≤ #(memberSubfamily a 𝒜).shatterer
ℬ : Finset (Finset α) := image (insert a) ((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
hℬ : #ℬ = #((memberSubfamily a 𝒜).shatterer ∩ (nonMemberSubfamily a 𝒜).shatterer)
s t : Finset α
ht : t.erase a ⊆ s
hs : (memberSubfamily a 𝒜).Shatters s ∧ (nonMemberSubfamily a 𝒜).Shatters s
ha : a ∉ t
u : Finset α
hu : u ∈ nonMemberSubfamily a 𝒜
hsu : s ∩ u = t.erase a
⊢ ∃ u ∈ 𝒜, insert a s ∩ u = t