-- Mathlib/MeasureTheory/Measure/ContinuousPreimage.lean
theorem tendsto_measure_symmDiff_preimage_nhds_zero
    {l : Filter α} {f : α → C(X, Y)} {g : C(X, Y)} {s : Set Y} (hfg : Tendsto f l (𝓝 g))
    (hf : ∀ᶠ a in l, MeasurePreserving (f a) μ ν) (hg : MeasurePreserving g μ ν)
    (hs : NullMeasurableSet s ν) (hνs : ν s ≠ ∞) :
    Tendsto (fun a ↦ μ ((f a ⁻¹' s) ∆ (g ⁻¹' s))) l (𝓝 0)

Proof state:
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
⊢ Tendsto (fun a => μ ((⇑(f a) ⁻¹' s) ∆ (⇑g ⁻¹' s))) l (𝓝 0)

Tactic:
have : ν.InnerRegularCompactLTTop := by
  rw [← hg.map_eq]
  exact .map_of_continuous (map_continuous _)

Result:
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this : ν.InnerRegularCompactLTTop
⊢ Tendsto (fun a => μ ((⇑(f a) ⁻¹' s) ∆ (⇑g ⁻¹' s))) l (𝓝 0)

Proof state:
case inr.intro.intro.intro
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hUs : ν (U ∆ s) < ε / 3
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

Tactic:
have hmU : NullMeasurableSet U ν := hUo.measurableSet.nullMeasurableSet

Result:
case inr.intro.intro.intro
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hUs : ν (U ∆ s) < ε / 3
hmU : NullMeasurableSet U ν
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

Proof state:
case intro.intro.intro.intro
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
this : ν.InnerRegularCompactLTTop
s : Set Y
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
ε : ℝ≥0∞
hε : ε > 0
hso : IsOpen s
hνs' : μ (⇑g ⁻¹' s) ≠ ⊤
K : Set X
hKg : MapsTo (⇑g) K s
hKco : IsCompact K
hKcl : IsClosed K
hKμ : μ (⇑g ⁻¹' s \ K) < ε / 2
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

Tactic:
have hKm : NullMeasurableSet K μ := hKcl.nullMeasurableSet

Result:
case intro.intro.intro.intro
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
this : ν.InnerRegularCompactLTTop
s : Set Y
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
ε : ℝ≥0∞
hε : ε > 0
hso : IsOpen s
hνs' : μ (⇑g ⁻¹' s) ≠ ⊤
K : Set X
hKg : MapsTo (⇑g) K s
hKco : IsCompact K
hKcl : IsClosed K
hKμ : μ (⇑g ⁻¹' s \ K) < ε / 2
hKm : NullMeasurableSet K μ
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

Proof state:
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ μ ((⇑(f a) ⁻¹' s) ∆ (⇑g ⁻¹' U)) + μ ((⇑g ⁻¹' U) ∆ (⇑g ⁻¹' s)) ≤
    μ ((⇑(f a) ⁻¹' s) ∆ (⇑(f a) ⁻¹' U)) + μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) + μ ((⇑g ⁻¹' U) ∆ (⇑g ⁻¹' s))

Tactic:
gcongr

Result:
case bc
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ μ ((⇑(f a) ⁻¹' s) ∆ (⇑g ⁻¹' U)) ≤ μ ((⇑(f a) ⁻¹' s) ∆ (⇑(f a) ⁻¹' U)) + μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U))