-- Mathlib/Combinatorics/Hall/Basic.lean
theorem Finset.all_card_le_biUnion_card_iff_exists_injective {ι : Type u} {α : Type v}
    [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
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : 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
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : 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 mpr.intro.intro.a
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : Injective f
hf₂ : ∀ (x : ι), f x ∈ t x
s : Finset ι
a✝ : α
⊢ a✝ ∈ image f s → a✝ ∈ s.biUnion t

Tactic:
rw [Finset.mem_image, Finset.mem_biUnion]

Result:
case mpr.intro.intro.a
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
f : ι → α
hf₁ : 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
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
i i' : ι
⊢ {i} ⊆ {i, i'}

Tactic:
simp

Result:
no goals

Proof state:
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
i i' : ι
subi : {i} ⊆ {i, i'}
⊢ {i'} ⊆ {i, i'}

Tactic:
simp

Result:
no goals

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

Tactic:
constructor

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

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

Proof state:
case mp
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
⊢ ∃ f, Injective f ∧ ∀ (x : ι), f x ∈ t x

Tactic:
haveI : ∀ ι' : (Finset ι)ᵒᵖ, Finite ((hallMatchingsFunctor t).obj ι') := by
  intro ι'
  rw [hallMatchingsFunctor]
  infer_instance

Result:
case mp
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
⊢ ∃ f, Injective f ∧ ∀ (x : ι), f x ∈ t x

Proof state:
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
⊢ ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')

Tactic:
intro ι'

Result:
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
ι' : (Finset ι)ᵒᵖ
⊢ Finite ((hallMatchingsFunctor t).obj ι')

Proof state:
case mp.intro
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
⊢ ∃ f, Injective f ∧ ∀ (x : ι), f x ∈ t x

Tactic:
refine ⟨?_, ?_, ?_⟩

Result:
case mp.intro.refine_1
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
⊢ ι → α

case mp.intro.refine_2
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
⊢ Injective ?mp.intro.refine_1

case mp.intro.refine_3
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
⊢ ∀ (x : ι), ?mp.intro.refine_1 x ∈ t x

Proof state:
case mp.intro.refine_2
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
i i' : ι
subi : {i} ≤ {i, i'}
subi' : {i'} ≤ {i, i'}
⊢ (fun i => ↑(u (Opposite.op {i})) ⟨i, ⋯⟩) i = (fun i => ↑(u (Opposite.op {i})) ⟨i, ⋯⟩) i' → i = i'

Tactic:
simp only

Result:
case mp.intro.refine_2
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
i i' : ι
subi : {i} ≤ {i, i'}
subi' : {i'} ≤ {i, i'}
⊢ ↑(u (Opposite.op {i})) ⟨i, ⋯⟩ = ↑(u (Opposite.op {i'})) ⟨i', ⋯⟩ → i = i'

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

Tactic:
intro h

Result:
case mp
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
⊢ ∃ f, Injective f ∧ ∀ (x : ι), f x ∈ t x

Proof state:
case mp.intro.refine_3
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
⊢ ∀ (x : ι), ↑(u (Opposite.op {x})) ⟨x, ⋯⟩ ∈ t x

Tactic:
intro i

Result:
case mp.intro.refine_3
ι : Type u
α : Type v
inst✝ : DecidableEq α
t : ι → Finset α
h : ∀ (s : Finset ι), #s ≤ #(s.biUnion t)
this✝ : ∀ (ι' : (Finset ι)ᵒᵖ), Nonempty ((hallMatchingsFunctor t).obj ι')
this : ∀ (ι' : (Finset ι)ᵒᵖ), Finite ((hallMatchingsFunctor t).obj ι')
u : (j : (Finset ι)ᵒᵖ) → (hallMatchingsFunctor t).obj j
hu : u ∈ (hallMatchingsFunctor t).sections
i : ι
⊢ ↑(u (Opposite.op {i})) ⟨i, ⋯⟩ ∈ t i