-- Lean 4 proof step from Mathlib.MeasureTheory.Constructions.HaarToSphere (goal)
Goal:
E : Type u_1
inst : NormedAddCommGroup E
inst_1 : NormedSpace ℝ E
inst_2 : MeasurableSpace E
μ : Measure E
inst_3 : BorelSpace E
s : Set (↑(sphere 0 1) : Type u_1)
hs : MeasurableSet s
rw :
  (HSMul.hSMul (β := Measure (↑(sphere 0 1) : Type u_1)) (dim E)
          (fst
            (restrict
              (comap
                (comp (β := { x : E // Membership.mem (γ := Set E) {0}ᶜ x })
                  (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) {0}ᶜ x))
                  (⇑(Homeomorph.symm (homeomorphUnitSphereProd E)) :
                    (a : (↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type)) → (↑{0}ᶜ : Type u_1)))
                μ)
              (SProd.sprod (α := Set (↑(sphere 0 1) : Type u_1)) univ
                (Iio (Subtype.mk (p := fun (x : ℝ) ↦ Membership.mem (γ := Set ℝ) (Ioi 0) x) 1 toSphere._proof_1))))) :
        (a : Set (↑(sphere 0 1) : Type u_1)) → ℝ≥0∞)
      s =
    HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((↑(dim E) : ℝ≥0∞) : ℝ≥0∞)
      ((μ : (a : Set E) → ℝ≥0∞)
          (HSMul.hSMul (α := Set ℝ) (Ioo 0 1)
            (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) (sphere 0 1) x) '' s)) :
        ℝ≥0∞)
rw₁ :
  HSMul.hSMul (β := ℝ≥0∞) (dim E)
      (((fst
                (restrict
                  (comap
                    (comp (β := { x : E // Membership.mem (γ := Set E) {0}ᶜ x })
                      (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) {0}ᶜ x))
                      (⇑(Homeomorph.symm (homeomorphUnitSphereProd E)) :
                        (a : (↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type)) → (↑{0}ᶜ : Type u_1)))
                    μ)
                  (SProd.sprod (α := Set (↑(sphere 0 1) : Type u_1)) univ
                    (Iio
                      (Subtype.mk (p := fun (x : ℝ) ↦ Membership.mem (γ := Set ℝ) (Ioi 0) x) 1 toSphere._proof_1)))) :
              Measure (↑(sphere 0 1) : Type u_1)) :
            (a : Set (↑(sphere 0 1) : Type u_1)) → ℝ≥0∞)
          s :
        ℝ≥0∞) =
    HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((↑(dim E) : ℝ≥0∞) : ℝ≥0∞)
      ((μ : (a : Set E) → ℝ≥0∞)
          (HSMul.hSMul (α := Set ℝ) (Ioo 0 1)
            (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) (sphere 0 1) x) '' s)) :
        ℝ≥0∞)
rw₂ :
  HSMul.hSMul (β := ℝ≥0∞) (dim E)
      (((restrict
                (comap
                  (comp (β := { x : E // Membership.mem (γ := Set E) {0}ᶜ x })
                    (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) {0}ᶜ x))
                    (⇑(Homeomorph.symm (homeomorphUnitSphereProd E)) :
                      (a : (↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type)) → (↑{0}ᶜ : Type u_1)))
                  μ)
                (SProd.sprod (α := Set (↑(sphere 0 1) : Type u_1)) univ
                  (Iio (Subtype.mk (p := fun (x : ℝ) ↦ Membership.mem (γ := Set ℝ) (Ioi 0) x) 1 toSphere._proof_1))) :
              Measure ((↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type))) :
            (a : Set ((↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type))) → ℝ≥0∞)
          (Prod.fst ⁻¹' s) :
        ℝ≥0∞) =
    HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((↑(dim E) : ℝ≥0∞) : ℝ≥0∞)
      ((μ : (a : Set E) → ℝ≥0∞)
          (HSMul.hSMul (α := Set ℝ) (Ioo 0 1)
            (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) (sphere 0 1) x) '' s)) :
        ℝ≥0∞)
⊢ ((μ.toSphere : Measure (↑(sphere 0 1) : Type u_1)) : (a : Set (↑(sphere 0 1) : Type u_1)) → ℝ≥0∞) s =
    HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((↑(dim E) : ℝ≥0∞) : ℝ≥0∞)
      ((μ : (a : Set E) → ℝ≥0∞)
          (HSMul.hSMul (α := Set ℝ) (Ioo 0 1)
            (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) (sphere 0 1) x) '' s)) :
        ℝ≥0∞)
have rw₃ : HSMul.hSMul (β := ℝ≥0∞) (dim E)
    (((comap
              (comp (β := { x : E // Membership.mem (γ := Set E) {0}ᶜ x })
                (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) {0}ᶜ x))
                (⇑(Homeomorph.symm (homeomorphUnitSphereProd E)) :
                  (a : (↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type)) → (↑{0}ᶜ : Type u_1)))
              μ :
            Measure ((↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type))) :
          (a : Set ((↑(sphere 0 1) : Type u_1) × (↑(Ioi 0) : Type))) → ℝ≥0∞)
        (Prod.fst ⁻¹' s ∩
          SProd.sprod (α := Set (↑(sphere 0 1) : Type u_1)) univ
            (Iio (Subtype.mk (p := fun (x : ℝ) ↦ Membership.mem (γ := Set ℝ) (Ioi 0) x) 1 toSphere._proof_1))) :
      ℝ≥0∞) =
  HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((↑(dim E) : ℝ≥0∞) : ℝ≥0∞)
    ((μ : (a : Set E) → ℝ≥0∞)
        (HSMul.hSMul (α := Set ℝ) (Ioo 0 1)
          (Subtype.val (p := fun (x : E) ↦ Membership.mem (γ := Set E) (sphere 0 1) x) '' s)) :
      ℝ≥0∞)
Tactic: rw [toSphere, smul_apply, fst_apply hs, restrict_apply (measurable_fst hs),
    ((MeasurableEmbedding.subtype_coe (measurableSet_singleton _).compl).comp
      (Homeomorph.measurableEmbedding _)).comap_apply,
    image_comp, Homeomorph.image_symm, univ_prod, ← Set.prod_eq, nsmul_eq_mul, toSphere_apply_aux]