-- Lean 4 proof step from Mathlib.RingTheory.Kaehler.TensorProduct (goal)
Goal:
R : Type u_1
S : Type u_2
A : Type u_3
B : Type u_4
inst : CommRing R
inst_1 : CommRing S
inst_2 : Algebra R S
inst_3 : CommRing A
inst_4 : CommRing B
inst_5 : Algebra R A
inst_6 : Algebra R B
inst_7 : Algebra A B
inst_8 : Algebra S B
inst_9 : IsScalarTower R A B
inst_10 : IsScalarTower R S B
h : Algebra.IsPushout R S A B
a b : B
zero :
  (IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) ⋯
          ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
        (a : B) → S ⊗[R] Ω[A⁄R])
      (0 * b) =
    HSMul.hSMul (α := B) (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) 0
        ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) ⋯
                ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
              (a : B) → S ⊗[R] Ω[A⁄R])
            b :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) +
      HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) b
        ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) ⋯
              ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
            (a : B) → S ⊗[R] Ω[A⁄R])
          0)
⊢ (IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) ⋯
          ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
        (a : B) → S ⊗[R] Ω[A⁄R])
      (a * b) =
    HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) a
        ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) ⋯
                ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
              (a : B) → S ⊗[R] Ω[A⁄R])
            b :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) +
      HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) b
        ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) ⋯
                ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
              (a : B) → S ⊗[R] Ω[A⁄R])
            a :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])
have smul : ∀ (x : S) (y : B)
  (e :
    (IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) sorry
            ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
          (a : B) → S ⊗[R] Ω[A⁄R])
        (y * b) =
      HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) y
          ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) sorry
                  ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
                (a : B) → S ⊗[R] Ω[A⁄R])
              b :
            TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) +
        HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) b
          ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) sorry
                  ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
                (a : B) → S ⊗[R] Ω[A⁄R])
              y :
            TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])),
  (IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) sorry
          ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
        (a : B) → S ⊗[R] Ω[A⁄R])
      (x • y * b) =
    HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) (x • y)
        ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) sorry
                ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
              (a : B) → S ⊗[R] Ω[A⁄R])
            b :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) +
      HSMul.hSMul (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) b
        ((IsBaseChange.lift (f := AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B)) sorry
                ((mk R S Ω[A⁄R] : (a : S) → Ω[A⁄R] →ₗ[R] S ⊗[R] Ω[A⁄R]) 1 ∘ₗ (↑(D R A) : A →ₗ[R] Ω[A⁄R])) :
              (a : B) → S ⊗[R] Ω[A⁄R])
            (x • y) :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])
Tactic: induction a using h.out.inductionOn with
    | zero => rw [map_zero, zero_smul, smul_zero, zero_add, zero_mul, map_zero]
    | smul x y e =>
      rw [smul_mul_assoc, map_smul, e, map_smul, smul_add,
        smul_comm x b, smul_assoc]
    | add b₁ b₂ e₁ e₂ => simp only [add_mul, add_smul, map_add, e₁, e₂, smul_add, add_add_add_comm]
    | tmul z =>
      dsimp
      induction b using h.out.inductionOn with
      | zero => rw [map_zero, zero_smul, smul_zero, zero_add, mul_zero, map_zero]
      | tmul =>
        simp only [AlgHom.toLinearMap_apply, IsScalarTower.coe_toAlgHom',
          algebraMap_smul, ← map_mul]
        rw [← IsScalarTower.toAlgHom_apply R, ← AlgHom.toLinearMap_apply, h.out.lift_eq,
          ← IsScalarTower.toAlgHom_apply R, ← AlgHom.toLinearMap_apply, h.out.lift_eq,
          ← IsScalarTower.toAlgHom_apply R, ← AlgHom.toLinearMap_apply, h.out.lift_eq]
        simp only [LinearMap.coe_comp, Derivation.coeFn_coe, Function.comp_apply,
          Derivation.leibniz, mk_apply, mulActionBaseChange_smul_tmul, TensorProduct.tmul_add]
      | smul _ _ e =>
        rw [mul_comm, smul_mul_assoc, map_smul, mul_comm, e,
            map_smul, smul_add, smul_comm, smul_assoc]
      | add _ _ e₁ e₂ => simp only [mul_add, add_smul, map_add, e₁, e₂, smul_add, add_add_add_comm]
Cleared hypotheses: a