-- 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
b : B
z : A
tmul :
  (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])
      (HMul.hMul (α := B) (β := B) ((algebraMap A B : (a : A) → B) z : B) (0 : B)) =
    HSMul.hSMul (α := B) (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) ((algebraMap A B : (a : A) → B) z : 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) +
      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])
            ((algebraMap A B : (a : A) → B) z) :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])
⊢ (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])
      (HMul.hMul (α := B) ((algebraMap A B : (a : A) → B) z : B) b) =
    HSMul.hSMul (α := B) (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) ((algebraMap A B : (a : A) → B) z : 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])
            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])
            ((algebraMap A B : (a : A) → B) z) :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])
have tmul₁ : ∀ (m : A),
  (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])
      (HMul.hMul (α := B) (β := B) ((algebraMap A B : (a : A) → B) z : B)
        ((AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B) : (a : A) → B) m : B)) =
    HSMul.hSMul (α := B) (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) ((algebraMap A B : (a : A) → B) z : 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])
            ((AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B) : (a : A) → B) m) :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]) +
      HSMul.hSMul (α := B) (β := TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])
        ((AlgHom.toLinearMap (IsScalarTower.toAlgHom R A B) : (a : A) → B) m : 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])
            ((algebraMap A B : (a : A) → B) z) :
          TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R])
Tactic: 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: b