-- Lean 4 proof step from Mathlib.RingTheory.DedekindDomain.Factorization (goal)
Goal:
R : Type u_1
inst : CommRing R
K : Type u_2
inst_1 : Field K
inst_2 : Algebra R K
inst_3 : IsFractionRing R K
inst_4 : IsDedekindDomain R
I : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K
hI : I ≠ 0
h :
  I =
    HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
      (spanSingleton R⁰
          ((algebraMap R K : (a : R) → K)
              (Classical.choose (p := fun (x : R) ↦
                ∃ (aI : Ideal R),
                  x ≠ 0 ∧
                    I =
                      HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                        (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                          FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                        ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                ⋯))⁻¹ :
        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
      ((↑(Classical.choose (p := fun (x : Ideal R) ↦
              Classical.choose (p := fun (x : R) ↦
                    ∃ (aI : Ideal R),
                      x ≠ 0 ∧
                        I =
                          HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                            FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                            (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                            ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                    ⋯ ≠
                  0 ∧
                I =
                  HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                    FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                    (spanSingleton R⁰
                        ((algebraMap R K : (a : R) → K)
                            (Classical.choose (p := fun (x : R) ↦
                              ∃ (aI : Ideal R),
                                x ≠ 0 ∧
                                  I =
                                    HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                                      FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                      (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                                        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                      ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                                        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                              ⋯))⁻¹ :
                      FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                    ((↑x : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
              ⋯) :
          FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
w : HeightOneSpectrum R
⊢ HPow.hPow (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (↑w.asIdeal : FractionalIdeal R⁰ K) (count K w I) =
    HPow.hPow (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (↑w.asIdeal : FractionalIdeal R⁰ K)
      (HSub.hSub (α := ℤ)
        (↑(Associates.count (Associates.mk w.asIdeal)
              (Associates.factors
                (Associates.mk
                  (Classical.choose (p := fun (x : Ideal R) ↦
                    Classical.choose (p := fun (x : R) ↦
                          ∃ (aI : Ideal R),
                            x ≠ 0 ∧
                              I =
                                HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                                  FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                  (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                                    FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                  ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                                    FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                          ⋯ ≠
                        0 ∧
                      I =
                        HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                          FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                          (spanSingleton R⁰
                              ((algebraMap R K : (a : R) → K)
                                  (Classical.choose (p := fun (x : R) ↦
                                    ∃ (aI : Ideal R),
                                      x ≠ 0 ∧
                                        I =
                                          HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                                            FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                            (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                            ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                                    ⋯))⁻¹ :
                            FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                          ((↑x : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                    ⋯)))) :
          ℤ)
        (↑(Associates.count (Associates.mk w.asIdeal)
              (Associates.factors
                (Associates.mk
                  (Ideal.span
                    {Classical.choose (p := fun (x : R) ↦
                        ∃ (aI : Ideal R),
                          x ≠ 0 ∧
                            I =
                              HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                                FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                                  FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                                  FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                        ⋯})))) :
          ℤ))
have h₁ : count K w I =
  HSub.hSub (α := ℤ)
    (↑(Associates.count (Associates.mk w.asIdeal)
          (Associates.factors
            (Associates.mk
              (Classical.choose (p := fun (x : Ideal R) ↦
                Classical.choose (p := fun (x : R) ↦
                      ∃ (aI : Ideal R),
                        x ≠ 0 ∧
                          I =
                            HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                              (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                                FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                              ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                                FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                      sorry ≠
                    0 ∧
                  I =
                    HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                      FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                      (spanSingleton R⁰
                          ((algebraMap R K : (a : R) → K)
                              (Classical.choose (p := fun (x : R) ↦
                                ∃ (aI : Ideal R),
                                  x ≠ 0 ∧
                                    I =
                                      HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                                        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                        (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                                          FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                                        ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                                          FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                                sorry))⁻¹ :
                        FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                      ((↑x : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                sorry)))) :
      ℤ)
    (↑(Associates.count (Associates.mk w.asIdeal)
          (Associates.factors
            (Associates.mk
              (Ideal.span
                {Classical.choose (p := fun (x : R) ↦
                    ∃ (aI : Ideal R),
                      x ≠ 0 ∧
                        I =
                          HMul.hMul (α := FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) (β :=
                            FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                            (spanSingleton R⁰ ((algebraMap R K : (a : R) → K) x)⁻¹ :
                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K)
                            ((↑aI : FractionalIdeal.{u_1, u_2} (R := R) R⁰ K) :
                              FractionalIdeal.{u_1, u_2} (R := R) R⁰ K))
                    sorry})))) :
      ℤ)
Tactic: apply congr_arg