-- Mathlib/MeasureTheory/Measure/Haar/Quotient.lean
theorem MeasureTheory.QuotientMeasureEqMeasurePreimage.haarMeasure_quotient [LocallyCompactSpace G]
    [QuotientMeasureEqMeasurePreimage ν μ] [i : HasFundamentalDomain Γ.op G ν]
    [IsFiniteMeasure μ] : IsHaarMeasure μ

Proof state:
case intro.mk.intro.ctop
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : LocallyCompactSpace G
inst✝¹ : QuotientMeasureEqMeasurePreimage ν μ
inst✝ : IsFiniteMeasure μ
K : PositiveCompacts G
K' : PositiveCompacts (G ⧸ Γ) := PositiveCompacts.map QuotientGroup.mk ⋯ ⋯ K
this : μ.IsMulLeftInvariant
s : Set G
finiteCovol : ν s ≠ ⊤
fund_dom_s : IsFundamentalDomain (↥Γ.op) s ν
⊢ ν (Quotient.mk (MulAction.orbitRel (↥Γ.op) G) ⁻¹' ↑K' ∩ s) ≠ ⊤

Tactic:
show ν (π ⁻¹' (π '' K) ∩ s) ≠ ⊤

Result:
case intro.mk.intro.ctop
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : LocallyCompactSpace G
inst✝¹ : QuotientMeasureEqMeasurePreimage ν μ
inst✝ : IsFiniteMeasure μ
K : PositiveCompacts G
K' : PositiveCompacts (G ⧸ Γ) := PositiveCompacts.map QuotientGroup.mk ⋯ ⋯ K
this : μ.IsMulLeftInvariant
s : Set G
finiteCovol : ν s ≠ ⊤
fund_dom_s : IsFundamentalDomain (↥Γ.op) s ν
⊢ ν (QuotientGroup.mk ⁻¹' (QuotientGroup.mk '' ↑K) ∩ s) ≠ ⊤

Proof state:
case intro
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : LocallyCompactSpace G
inst✝¹ : QuotientMeasureEqMeasurePreimage ν μ
i : HasFundamentalDomain (↥Γ.op) G ν
inst✝ : IsFiniteMeasure μ
K : PositiveCompacts G
K' : PositiveCompacts (G ⧸ Γ) := PositiveCompacts.map QuotientGroup.mk ⋯ ⋯ K
this : μ.IsMulLeftInvariant
finiteCovol : covolume (↥Γ.op) G ν ≠ ⊤
⊢ (μ ↑K' • haarMeasure K').IsHaarMeasure

Tactic:
obtain ⟨s, fund_dom_s⟩ := i

Result:
case intro.mk.intro
G : Type u_1
inst✝¹⁴ : Group G
inst✝¹³ : MeasurableSpace G
inst✝¹² : TopologicalSpace G
inst✝¹¹ : IsTopologicalGroup G
inst✝¹⁰ : BorelSpace G
inst✝⁹ : PolishSpace G
Γ : Subgroup G
inst✝⁸ : Γ.Normal
inst✝⁷ : T2Space (G ⧸ Γ)
inst✝⁶ : SecondCountableTopology (G ⧸ Γ)
μ : Measure (G ⧸ Γ)
inst✝⁵ : Countable ↥Γ
ν : Measure G
inst✝⁴ : ν.IsHaarMeasure
inst✝³ : ν.IsMulRightInvariant
inst✝² : LocallyCompactSpace G
inst✝¹ : QuotientMeasureEqMeasurePreimage ν μ
inst✝ : IsFiniteMeasure μ
K : PositiveCompacts G
K' : PositiveCompacts (G ⧸ Γ) := PositiveCompacts.map QuotientGroup.mk ⋯ ⋯ K
this : μ.IsMulLeftInvariant
finiteCovol : covolume (↥Γ.op) G ν ≠ ⊤
s : Set G
fund_dom_s : IsFundamentalDomain (↥Γ.op) s ν
⊢ (μ ↑K' • haarMeasure K').IsHaarMeasure