-- Lean 4 proof step from Mathlib.LinearAlgebra.FreeModule.Int (goal)
Goal:
ι : Type u_1
R : Type u_2
M : Type u_3
n : ℕ
inst : CommRing R
inst_1 : AddCommGroup M
inst_2 : Fintype ι
inst_3 : Module R M
N : Submodule R M
snf : SmithNormalForm N ι n
rcases :
  ∀ (bM : Basis ι R M) (bN : Basis.{0, u_2, u_3} (Fin n) R ↥N) (f : Fin n ↪ ι) (a : Fin n → R)
    (snf :
      ∀ (i : Fin n),
        (↑((bN : (a : Fin n) → ↥N) i) : M) =
          HSMul.hSMul (β := M) (a i) ((bM : (a : ι) → M) ((f : (a : Fin n) → ι) i : ι) : M)),
    N.toAddSubgroup.index =
      Nat.card R ^ (Fintype.card ι - n) *
        ∏ i : Fin n,
          AddSubgroup.index.{u_2} (G := R)
            (Submodule.toAddSubgroup
              (Ideal.span {{ bM := bM, bN := bN, f := f, a := a, snf := snf : SmithNormalForm N ι n }.a i}))
rcases₁ :
  ∀ (bM : Basis ι R M) (bN : Basis.{0, u_2, u_3} (Fin n) R ↥N) (f : Fin n ↪ ι) (a : Fin n → R)
    (snf :
      ∀ (i : Fin n),
        (↑((bN : (a : Fin n) → ↥N) i) : M) =
          HSMul.hSMul (β := M) (a i) ((bM : (a : ι) → M) ((f : (a : Fin n) → ι) i : ι) : M)),
    N.toAddSubgroup.index =
      Nat.card R ^ (Fintype.card ι - n) *
        ∏ i : Fin n, AddSubgroup.index.{u_2} (G := R) (Submodule.toAddSubgroup (Ideal.span {a i}))
rcases₂ rcases₃ :
  ∀ (bM : Basis ι R M) (bN : Basis.{0, u_2, u_3} (Fin n) R ↥N) (f : Fin n ↪ ι) (a : Fin n → R)
    (snf :
      ∀ (i : Fin n),
        (↑((bN : (a : Fin n) → ↥N) i) : M) =
          HSMul.hSMul (β := M) (a i) ((bM : (a : ι) → M) ((f : (a : Fin n) → ι) i : ι) : M)),
    let N' := Submodule.map (↑bM.equivFun : M →ₗ[R] ι → R) N;
    ∀ (hN' : N' = Submodule.map (↑bM.equivFun : M →ₗ[R] ι → R) N),
      N.toAddSubgroup.index =
        Nat.card R ^ (Fintype.card ι - n) *
          ∏ i : Fin n, AddSubgroup.index.{u_2} (G := R) (Submodule.toAddSubgroup (Ideal.span {a i}))
⊢ N.toAddSubgroup.index =
    Nat.card R ^ (Fintype.card ι - n) *
      ∏ i : Fin n, AddSubgroup.index.{u_2} (G := R) (Submodule.toAddSubgroup (Ideal.span {snf.a i}))
have rcases₄ : ∀ (bM : Basis ι R M) (bN : Basis.{0, u_2, u_3} (Fin n) R ↥N) (f : Fin n ↪ ι) (a : Fin n → R)
  (snf :
    ∀ (i : Fin n),
      (↑((bN : (a : Fin n) → ↥N) i) : M) =
        HSMul.hSMul (β := M) (a i) ((bM : (a : ι) → M) ((f : (a : Fin n) → ι) i : ι) : M)),
  let N' := Submodule.map (↑bM.equivFun : M →ₗ[R] ι → R) N;
  ∀ (hN' : N' = Submodule.map (↑bM.equivFun : M →ₗ[R] ι → R) N),
    let bN' := bN.map (bM.equivFun.submoduleMap N);
    ∀
      (snf' :
        ∀ (i : Fin n),
          Eq (α := ι → R) ((↑((bN' : (a : Fin n) → ↥N') i) : ι → R) : ι → R)
            (Pi.single (M := fun (j : ι) ↦ R) ((f : (a : Fin n) → ι) i) (a i) : (j : ι) → R)),
      N.toAddSubgroup.index =
        Nat.card R ^ (Fintype.card ι - n) *
          ∏ i : Fin n, AddSubgroup.index.{u_2} (G := R) (Submodule.toAddSubgroup (Ideal.span {a i}))
Tactic: classical
  rcases snf with ⟨bM, bN, f, a, snf⟩
  dsimp only
  set N' : Submodule R (ι → R) := N.map bM.equivFun.toLinearMap with hN'
  let bN' : Basis (Fin n) R N' := bN.map (bM.equivFun.submoduleMap N)
  have snf' : ∀ i, (bN' i : ι → R) = Pi.single (f i) (a i) := by
    intro i
    simp only [map_apply, bN']
    rw [LinearEquiv.submoduleMap_apply]
    simp only [equivFun_apply, snf, map_smul, repr_self, Finsupp.single_eq_pi_single]
    ext j
    simp [Pi.single_apply]
  have hNN' : N.toAddSubgroup.index = N'.toAddSubgroup.index := by
    set e : (ι → R) ≃+ M := ↑bM.equivFun.symm with he
    let e' : (ι → R) →+ M := e
    have he' : Function.Surjective e' := e.surjective
    convert! (AddSubgroup.index_comap_of_surjective N.toAddSubgroup he').symm using 2
    rw [AddSubgroup.comap_equiv_eq_map_symm, he, hN', LinearEquiv.coe_toAddEquiv_symm,
    AddEquiv.symm_symm]
    exact Submodule.map_toAddSubgroup ..
  rw [hNN']
  have hN' : N'.toAddSubgroup = AddSubgroup.pi Set.univ
      (fun i ↦ (Ideal.span {if h : ∃ j, f j = i then a h.choose else 0}).toAddSubgroup) := by
    ext g
    simp only [Submodule.mem_toAddSubgroup, bN'.mem_submodule_iff', snf', AddSubgroup.mem_pi,
      Set.mem_univ, true_implies, Ideal.mem_span_singleton]
    refine ⟨fun h ↦ ?_, fun h ↦ ?_⟩
    · rcases h with ⟨c, rfl⟩
      intro i
      simp only [Finset.sum_apply, Pi.smul_apply, Pi.single_apply]
      split_ifs with h
      · convert! dvd_mul_left (a h.choose) (c h.choose)
        calc ∑ x : Fin n, _ = c h.choose * if i = f h.choose then a h.choose else 0 := by
              refine Finset.sum_eq_single h.choose ?_ (by simp)
              rintro j - hj
              have hinj := f.injective.ne hj
              rw [h.choose_spec] at hinj
              simp [hinj.symm]
          _ = c h.choose * a h.choose := by simp [h.choose_spec]
      · convert! dvd_refl (0 : R)
        convert! Finset.sum_const_zero with j
        rw [not_exists] at h
        specialize h j
        rw [eq_comm] at h
        simp [h]
    · refine ⟨fun j ↦ (h (f j)).choose, ?_⟩
      ext i
      simp only [EmbeddingLike.apply_eq_iff_eq, exists_eq, ↓reduceDIte, Classical.choose_eq,
        Finset.sum_apply, Pi.smul_apply, Pi.single_apply, smul_ite, smul_zero]
      rw [eq_comm]
      by_cases! hj : ∃ j, f j = i
      · calc ∑ x : Fin n, _ =
            if i = f hj.choose then (h (f hj.choose)).choose * a hj.choose else 0 := by
              convert! Finset.sum_eq_single (M := R) hj.choose ?_ ?_
              · simp
              · rintro j - h
                have hinj := f.injective.ne h
                rw [hj.choose_spec] at hinj
                simp [hinj.symm]
              · simp
          _ = g i := by
              simp only [hj.choose_spec, ↓reduceIte]
              rw [mul_comm]
              conv_rhs =>
                rw [← hj.choose_spec, (h (f hj.choose)).choose_spec]
              simp only [EmbeddingLike.apply_eq_iff_eq, exists_eq, ↓reduceDIte, Classical.choose_eq]
              congr!
              · exact hj.choose_spec.symm
              · simp [hj]
      · convert! Finset.sum_const_zero with x
        · specialize hj x
          rw [ne_comm] at hj
          simp [hj]
        · rw [← zero_dvd_iff]
          convert! h i
          simp [hj]
  simp only [hN', AddSubgroup.index_pi, apply_dite, Finset.prod_dite, Set.singleton_zero,
    Ideal.span_zero, Submodule.bot_toAddSubgroup, AddSubgroup.index_pi, AddSubgroup.index_bot,
    Finset.prod_const, Finset.univ_eq_attach, Finset.card_attach]
  rw [mul_comm]
  congr
  · convert! Finset.card_compl {x | ∃ j, f j = x} using 2
    · exact (Finset.compl_filter _).symm
    · convert! (Finset.card_image_of_injective Finset.univ f.injective).symm <;> simp
  · rw [Finset.attach_eq_univ]
    let f' : Fin n → { x // x ∈ Finset.filter (fun x ↦ ∃ j, f j = x) Finset.univ } :=
      fun i ↦ ⟨f i, by simp⟩
    have hf' : Function.Injective f' := fun i j hij ↦ by
      rw [Subtype.ext_iff] at hij
      exact f.injective hij
    let f'' : Fin n ↪ { x // x ∈ Finset.filter (fun x ↦ ∃ j, f j = x) Finset.univ } :=
      ⟨f', hf'⟩
    have hu : (Finset.univ : Finset { x // x ∈ Finset.filter (fun x ↦ ∃ j, f j = x) Finset.univ }) =
      Finset.univ.map f'' := by
      ext x
      simp only [Finset.univ_eq_attach, Finset.mem_attach, Finset.mem_map, Finset.mem_univ,
        true_and, true_iff]
      have hx := x.property
      simp only [Finset.univ_filter_exists, Finset.mem_image, Finset.mem_univ, true_and] at hx
      rcases hx with ⟨i, hi⟩
      refine ⟨i, ?_⟩
      rw [Subtype.ext_iff]
      exact hi
    rw [hu, Finset.prod_map]
    congr! with i
    rw [← f.injective.eq_iff]
    generalize_proofs h
    rw [h.choose_spec]
    rfl
Cleared hypotheses: snf