-- Mathlib/MeasureTheory/Group/AddCircle.lean
theorem volume_of_add_preimage_eq (s I : Set <| AddCircle T) (u x : AddCircle T)
    (hu : IsOfFinAddOrder u) (hs : (u +ᵥ s : Set <| AddCircle T) =ᵐ[volume] s)
    (hI : I =ᵐ[volume] ball x (T / (2 * addOrderOf u))) :
    volume s = addOrderOf u • volume (s ∩ I)

Proof state:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
hsG : ∀ (g : ↥G), g +ᵥ s =ᶠ[ae volume] s
⊢ volume s = addOrderOf u • volume (s ∩ I)

Tactic:
rw [(isAddFundamentalDomain_of_ae_ball I u x hu hI).measure_eq_card_smul_of_vadd_ae_eq_self s hsG,
  ← Nat.card_zmultiples u]

Result:
no goals

Proof state:
case mk
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
y : AddCircle T
hy : y ∈ G
⊢ ⟨y, hy⟩ +ᵥ s =ᶠ[ae volume] s

Tactic:
exact (vadd_ae_eq_self_of_mem_zmultiples hs hy :)

Result:
no goals

Proof state:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
⊢ ∀ (g : ↥G), g +ᵥ s =ᶠ[ae volume] s

Tactic:
rintro ⟨y, hy⟩

Result:
case mk
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
y : AddCircle T
hy : y ∈ G
⊢ ⟨y, hy⟩ +ᵥ s =ᶠ[ae volume] s

Proof state:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
⊢ volume s = addOrderOf u • volume (s ∩ I)

Tactic:
have hsG : ∀ g : G, (g +ᵥ s : Set <| AddCircle T) =ᵐ[volume] s := by
  rintro ⟨y, hy⟩; exact (vadd_ae_eq_self_of_mem_zmultiples hs hy :)

Result:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
hsG : ∀ (g : ↥G), g +ᵥ s =ᶠ[ae volume] s
⊢ volume s = addOrderOf u • volume (s ∩ I)

Proof state:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
⊢ volume s = addOrderOf u • volume (s ∩ I)

Tactic:
haveI : Fintype G := @Fintype.ofFinite _ hu.finite_zmultiples.to_subtype

Result:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
this : Fintype ↥G
⊢ volume s = addOrderOf u • volume (s ∩ I)

Proof state:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
⊢ volume s = addOrderOf u • volume (s ∩ I)

Tactic:
let G := AddSubgroup.zmultiples u

Result:
T : ℝ
hT : Fact (0 < T)
s I : Set (AddCircle T)
u x : AddCircle T
hu : IsOfFinAddOrder u
hs : u +ᵥ s =ᶠ[ae volume] s
hI : I =ᶠ[ae volume] ball x (T / (2 * ↑(addOrderOf u)))
G : AddSubgroup (AddCircle T) := AddSubgroup.zmultiples u
⊢ volume s = addOrderOf u • volume (s ∩ I)