-- Mathlib/MeasureTheory/Group/FundamentalDomain.lean
theorem mk_of_measure_univ_le [IsFiniteMeasure μ] [Countable G] (h_meas : NullMeasurableSet s μ)
    (h_ae_disjoint : ∀ g ≠ (1 : G), AEDisjoint μ (g • s) s)
    (h_qmp : ∀ g : G, QuasiMeasurePreserving (g • · : α → α) μ μ)
    (h_measure_univ_le : μ (univ : Set α) ≤ ∑' g : G, μ (g • s)) : IsFundamentalDomain G s μ :=
  have aedisjoint : Pairwise (AEDisjoint μ on fun g : G => g • s) :=
    pairwise_aedisjoint_of_aedisjoint_forall_ne_one h_ae_disjoint h_qmp
  { nullMeasurableSet := h_meas
    aedisjoint
    ae_covers

Proof state:
G : Type u_1
α : Type u_3
inst✝⁴ : Group G
inst✝³ : MulAction G α
inst✝² : MeasurableSpace α
s : Set α
μ : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : Countable G
h_ae_disjoint : ∀ (g : G), g ≠ 1 → AEDisjoint μ (g • s) s
h_qmp : ∀ (g : G), QuasiMeasurePreserving (fun x => g • x) μ μ
h_measure_univ_le : μ univ ≤ ∑' (g : G), μ (g • s)
aedisjoint : Pairwise (AEDisjoint μ on fun g => g • s)
h_meas : ∀ (g : G), NullMeasurableSet (g • s) μ
h_meas' : NullMeasurableSet {a | ∃ g, g • a ∈ s} μ
⊢ ∀ᵐ (x : α) ∂μ, ∃ g, g • x ∈ s

Tactic:
rw [ae_iff_measure_eq h_meas', ← iUnion_smul_eq_setOf_exists]

Result:
G : Type u_1
α : Type u_3
inst✝⁴ : Group G
inst✝³ : MulAction G α
inst✝² : MeasurableSpace α
s : Set α
μ : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : Countable G
h_ae_disjoint : ∀ (g : G), g ≠ 1 → AEDisjoint μ (g • s) s
h_qmp : ∀ (g : G), QuasiMeasurePreserving (fun x => g • x) μ μ
h_measure_univ_le : μ univ ≤ ∑' (g : G), μ (g • s)
aedisjoint : Pairwise (AEDisjoint μ on fun g => g • s)
h_meas : ∀ (g : G), NullMeasurableSet (g • s) μ
h_meas' : NullMeasurableSet {a | ∃ g, g • a ∈ s} μ
⊢ μ (⋃ g, g • s) = μ univ

Proof state:
G : Type u_1
α : Type u_3
inst✝⁴ : Group G
inst✝³ : MulAction G α
inst✝² : MeasurableSpace α
s : Set α
μ : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : Countable G
h_meas : NullMeasurableSet s μ
h_ae_disjoint : ∀ (g : G), g ≠ 1 → AEDisjoint μ (g • s) s
h_qmp : ∀ (g : G), QuasiMeasurePreserving (fun x => g • x) μ μ
h_measure_univ_le : μ univ ≤ ∑' (g : G), μ (g • s)
aedisjoint : Pairwise (AEDisjoint μ on fun g => g • s)
g : G
⊢ NullMeasurableSet ((fun x => g⁻¹ • x) ⁻¹' s) μ

Tactic:
exact h_meas.preimage (h_qmp g⁻¹)

Result:
no goals

Proof state:
G : Type u_1
α : Type u_3
inst✝⁴ : Group G
inst✝³ : MulAction G α
inst✝² : MeasurableSpace α
s : Set α
μ : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : Countable G
h_ae_disjoint : ∀ (g : G), g ≠ 1 → AEDisjoint μ (g • s) s
h_qmp : ∀ (g : G), QuasiMeasurePreserving (fun x => g • x) μ μ
h_measure_univ_le : μ univ ≤ ∑' (g : G), μ (g • s)
aedisjoint : Pairwise (AEDisjoint μ on fun g => g • s)
h_meas : ∀ (g : G), NullMeasurableSet (g • s) μ
⊢ NullMeasurableSet {a | ∃ g, g • a ∈ s} μ

Tactic:
rw [← iUnion_smul_eq_setOf_exists]

Result:
G : Type u_1
α : Type u_3
inst✝⁴ : Group G
inst✝³ : MulAction G α
inst✝² : MeasurableSpace α
s : Set α
μ : Measure α
inst✝¹ : IsFiniteMeasure μ
inst✝ : Countable G
h_ae_disjoint : ∀ (g : G), g ≠ 1 → AEDisjoint μ (g • s) s
h_qmp : ∀ (g : G), QuasiMeasurePreserving (fun x => g • x) μ μ
h_measure_univ_le : μ univ ≤ ∑' (g : G), μ (g • s)
aedisjoint : Pairwise (AEDisjoint μ on fun g => g • s)
h_meas : ∀ (g : G), NullMeasurableSet (g • s) μ
⊢ NullMeasurableSet (⋃ g, g • s) μ