-- Module: Mathlib.RingTheory.DedekindDomain.Factorization
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
⊢ 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)

Tactic:
have h := (Classical.choose_spec (Classical.choose_spec (exists_eq_spanSingleton_mul I))).2