-- Mathlib/Combinatorics/Hall/Finite.lean
theorem Finset.all_card_le_biUnion_card_iff_existsInjective' {ι α : Type*} [Finite ι]
    [DecidableEq α] (t : ι → Finset α) :
    (∀ s : Finset ι, #s ≤ #(s.biUnion t)) ↔
      ∃ f : ι → α, Function.Injective f ∧ ∀ x, f x ∈ t x

Proof state:
case mpr.intro.intro.a
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
a✝ : α
⊢ (∃ a ∈ s, f a = a✝) → ∃ a ∈ s, a✝ ∈ t a

Tactic:
rintro ⟨x, hx, rfl⟩

Result:
case mpr.intro.intro.a.intro.intro
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
x : ι
hx : x ∈ s
⊢ ∃ a ∈ s, f x ∈ t a

Proof state:
case mpr.intro.intro
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
⊢ #(image f s) ≤ #(s.biUnion t)

Tactic:
apply card_le_card

Result:
case mpr.intro.intro.a
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
⊢ image f s ⊆ s.biUnion t

Proof state:
case mpr.intro.intro
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
⊢ #s ≤ #(s.biUnion t)

Tactic:
rw [← card_image_of_injective s hf₁]

Result:
case mpr.intro.intro
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
⊢ #(image f s) ≤ #(s.biUnion t)

Proof state:
case mpr.intro.intro.a
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
⊢ image f s ⊆ s.biUnion t

Tactic:
intro

Result:
case mpr.intro.intro.a
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
a✝ : α
⊢ a✝ ∈ image f s → a✝ ∈ s.biUnion t

Proof state:
case mp
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
⊢ (∀ (s : Finset ι), #s ≤ #(s.biUnion t)) → ∃ f, Function.Injective f ∧ ∀ (x : ι), f x ∈ t x

Tactic:
exact HallMarriageTheorem.hall_hard_inductive

Result:
no goals

Proof state:
case mpr.intro.intro.a
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
a✝ : α
⊢ a✝ ∈ image f s → a✝ ∈ s.biUnion t

Tactic:
rw [mem_image, mem_biUnion]

Result:
case mpr.intro.intro.a
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
a✝ : α
⊢ (∃ a ∈ s, f a = a✝) → ∃ a ∈ s, a✝ ∈ t a

Proof state:
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
⊢ (∀ (s : Finset ι), #s ≤ #(s.biUnion t)) ↔ ∃ f, Function.Injective f ∧ ∀ (x : ι), f x ∈ t x

Tactic:
constructor

Result:
case mp
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
⊢ (∀ (s : Finset ι), #s ≤ #(s.biUnion t)) → ∃ f, Function.Injective f ∧ ∀ (x : ι), f x ∈ t x

case mpr
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
⊢ (∃ f, Function.Injective f ∧ ∀ (x : ι), f x ∈ t x) → ∀ (s : Finset ι), #s ≤ #(s.biUnion t)

Proof state:
case mpr
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
⊢ (∃ f, Function.Injective f ∧ ∀ (x : ι), f x ∈ t x) → ∀ (s : Finset ι), #s ≤ #(s.biUnion t)

Tactic:
rintro ⟨f, hf₁, hf₂⟩ s

Result:
case mpr.intro.intro
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
⊢ #s ≤ #(s.biUnion t)

Proof state:
case mpr.intro.intro.a.intro.intro
ι : Type u_1
α : Type u_2
inst✝¹ : Finite ι
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Function.Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
x : ι
hx : x ∈ s
⊢ ∃ a ∈ s, f x ∈ t a

Tactic:
exact ⟨x, hx, hf₂ x⟩

Result:
no goals