-- Lean 4 proof step from Mathlib.RingTheory.PicardGroup (goal)
Goal:
R : Type u
A : Type u_4
inst : CommSemiring R
inst_1 : Semiring A
inst_2 : Algebra R A
inst_3 : FaithfulSMul R A
I : (Submodule R A)ˣ
T T' : Finset A
hT : (↑T : Set A) ⊆ (↑(↑I⁻¹ : Submodule R A) : Set A)
hT' : (↑T' : Set A) ⊆ (↑(↑I : Submodule R A) : Set A)
one_mem : Membership.mem (γ := Submodule R A) (span R ((↑T : Set A) * (↑T' : Set A))) 1
rw :
  ∀
    (one_mem :
      Membership.mem (γ := Submodule R A) (span R (Set.image2 (fun (x1 x2 : A) ↦ x1 * x2) (↑T : Set A) (↑T' : Set A)))
        1),
    Projective.{u, u_4} R ↥(↑I : Submodule R A) ∧
      Function.Bijective (α := A ⊗[R] ↥(↑I : Submodule R A)) (β := A)
        (⇑(mul' R A ∘ₗ lTensor A (↑I : Submodule R A).subtype) : (a : A ⊗[R] ↥(↑I : Submodule R A)) → A)
rw₁ :
  ∀
    (one_mem :
      Membership.mem (γ := Submodule R A) (span R (↑(Finset.image₂ (fun (x1 x2 : A) ↦ x1 * x2) T T') : Set A)) 1),
    Projective.{u, u_4} R ↥(↑I : Submodule R A) ∧
      Function.Bijective (α := A ⊗[R] ↥(↑I : Submodule R A)) (β := A)
        (⇑(mul' R A ∘ₗ lTensor A (↑I : Submodule R A).subtype) : (a : A ⊗[R] ↥(↑I : Submodule R A)) → A)
rw₂ rw₃ rw₄ :
  ∀
    (one_mem :
      ∃ (f : A → R),
        Function.support f ⊆ (↑(Finset.image₂ (fun (x1 x2 : A) ↦ x1 * x2) T T') : Set A) ∧
          Eq (α := A) (∑ a ∈ Finset.image₂ (fun (x1 x2 : A) ↦ x1 * x2) T T', f a • a) 1),
    Projective.{u, u_4} R ↥(↑I : Submodule R A) ∧
      Function.Bijective (α := A ⊗[R] ↥(↑I : Submodule R A)) (β := A)
        (⇑(mul' R A ∘ₗ lTensor A (↑I : Submodule R A).subtype) : (a : A ⊗[R] ↥(↑I : Submodule R A)) → A)
rw₅ :
  let S : Finset A := Finset.image₂ (fun (x1 x2 : A) ↦ x1 * x2) T T';
  ∀ (one_mem : ∃ (f : A → R), Function.support f ⊆ (↑S : Set A) ∧ Eq (α := A) (∑ a ∈ S, f a • a) 1),
    Projective.{u, u_4} R ↥(↑I : Submodule R A) ∧
      Function.Bijective (α := A ⊗[R] ↥(↑I : Submodule R A)) (β := A)
        (⇑(mul' R A ∘ₗ lTensor A (↑I : Submodule R A).subtype) : (a : A ⊗[R] ↥(↑I : Submodule R A)) → A)
⊢ Projective.{u, u_4} R ↥(↑I : Submodule R A) ∧
    Function.Bijective (α := A ⊗[R] ↥(↑I : Submodule R A)) (β := A)
      (⇑(mul' R A ∘ₗ lTensor A (↑I : Submodule R A).subtype) : (a : A ⊗[R] ↥(↑I : Submodule R A)) → A)
have rw₆ : let S : Finset A := Finset.image₂ (fun (x1 x2 : A) ↦ x1 * x2) T T';
∀ (r : A → R) (hr : Function.support r ⊆ (↑S : Set A) ∧ Eq (α := A) (∑ a ∈ S, r a • a) 1),
  Projective.{u, u_4} R ↥(↑I : Submodule R A) ∧
    Function.Bijective (α := A ⊗[R] ↥(↑I : Submodule R A)) (β := A)
      (⇑(mul' R A ∘ₗ lTensor A (↑I : Submodule R A).subtype) : (a : A ⊗[R] ↥(↑I : Submodule R A)) → A)
Tactic: classical
  rw [← Set.image2_mul, ← Finset.coe_image₂, mem_span_finset] at one_mem
  set S := T.image₂ (· * ·) T'
  obtain ⟨r, hr⟩ := one_mem
  choose a ha b hb eq using fun i : S ↦ Finset.mem_image₂.mp i.2
  let f : I →ₗ[R] S → R := .pi fun i ↦ (LinearEquiv.ofInjective
      (Algebra.linearMap R A) (FaithfulSMul.algebraMap_injective R A)).symm.comp <|
    restrict (mulRight R (r i • a i)) fun x hx ↦ by
      rw [← one_eq_range, ← I.mul_inv]; exact mul_mem_mul hx (I⁻¹.1.smul_mem _ <| hT <| ha i)
  have hf (x : I.1) (i : S) : algebraMap R A (f x i) = x * r i • a i := by
    dsimp [f, ← Algebra.linearMap_apply]
    exact LinearEquiv.ofInjective_symm_apply ..
  let g : (S → R) →ₗ[R] I := .lsum _ _ ℕ fun i ↦ .toSpanSingleton _ _ ⟨b i, hT' <| hb i⟩
  have hgf : g ∘ₗ f = .id := LinearMap.ext fun x ↦ Subtype.ext <| by
    simp only [g, lsum_apply, comp_apply, sum_apply, toSpanSingleton_apply, proj_apply]
    simp_rw [coe_sum, coe_smul, Algebra.smul_def, hf, mul_assoc, ← Finset.mul_sum,
      Algebra.smul_mul_assoc, eq, (Finset.sum_coe_sort ..).trans hr.2, mul_one, id_apply]
  set m := mul' R A ∘ₗ I.1.subtype.lTensor A
  have eq : (piScalarRight R R A S).toLinearMap ∘ₗ f.lTensor A =
      (.pi fun i : S ↦ mulRight R (r i • a i)) ∘ₗ m := by
    ext; simp [(Algebra.smul_def ..).trans (Algebra.commutes ..), hf, m, mul_assoc]
  have := (piScalarRight R R A S).injective.comp <| injective_of_comp_eq_id
    (f.lTensor A) (g.lTensor A) <| by rw [← lTensor_comp, hgf, lTensor_id]
  rw [← LinearEquiv.coe_toLinearMap, ← coe_comp, eq, coe_comp] at this
  refine ⟨.of_split f g hgf, .of_comp this, range_eq_top.mp ?_⟩
  rw [show m = mulMap ⊤ I ∘ₗ (topEquiv.symm.rTensor I.1).toLinearMap by ext; rfl, range_comp,
    LinearEquiv.range, map_top, mulMap_range, top_mul_eq_top_of_mul_eq_one I.inv_mul]
Cleared hypotheses: one_mem