-- 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
x : TensorProduct.{u_1, u_2, u_3} R S Ω[A⁄R]
zero :
  (LinearMap.liftBaseChange S
          ((↑R :
              {S M : Type u_3} →
                {M₂ : Type u_4} →
                  [inst : Semiring R] →
                    [inst_11 : Semiring S] →
                      [inst_12 : AddCommMonoid M] →
                        [inst_13 : AddCommMonoid M₂] →
                          [inst_14 : Module R M] →
                            [inst_15 : Module R M₂] →
                              [inst_16 : Module S M] →
                                [inst_17 : Module S M₂] →
                                  [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
            (map R S A B)) :
        (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
      (HSMul.hSMul (α := B) 0 x) =
    HSMul.hSMul (α := B) (β := Ω[B⁄S]) 0
      ((LinearMap.liftBaseChange S
              ((↑R :
                  {S M : Type u_3} →
                    {M₂ : Type u_4} →
                      [inst : Semiring R] →
                        [inst_11 : Semiring S] →
                          [inst_12 : AddCommMonoid M] →
                            [inst_13 : AddCommMonoid M₂] →
                              [inst_14 : Module R M] →
                                [inst_15 : Module R M₂] →
                                  [inst_16 : Module S M] →
                                    [inst_17 : Module S M₂] →
                                      [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
                (map R S A B)) :
            (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
          x :
        Ω[B⁄S])
⊢ (LinearMap.liftBaseChange S
          ((↑R :
              {S M : Type u_3} →
                {M₂ : Type u_4} →
                  [inst : Semiring R] →
                    [inst_11 : Semiring S] →
                      [inst_12 : AddCommMonoid M] →
                        [inst_13 : AddCommMonoid M₂] →
                          [inst_14 : Module R M] →
                            [inst_15 : Module R M₂] →
                              [inst_16 : Module S M] →
                                [inst_17 : Module S M₂] →
                                  [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
            (map R S A B)) :
        (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
      (b • x) =
    HSMul.hSMul (β := Ω[B⁄S]) b
      ((LinearMap.liftBaseChange S
              ((↑R :
                  {S M : Type u_3} →
                    {M₂ : Type u_4} →
                      [inst : Semiring R] →
                        [inst_11 : Semiring S] →
                          [inst_12 : AddCommMonoid M] →
                            [inst_13 : AddCommMonoid M₂] →
                              [inst_14 : Module R M] →
                                [inst_15 : Module R M₂] →
                                  [inst_16 : Module S M] →
                                    [inst_17 : Module S M₂] →
                                      [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
                (map R S A B)) :
            (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
          x :
        Ω[B⁄S])
have smul : ∀ (s : S) (b : B)
  (e :
    (LinearMap.liftBaseChange S
            ((↑R :
                {S M : Type u_3} →
                  {M₂ : Type u_4} →
                    [inst : Semiring R] →
                      [inst_11 : Semiring S] →
                        [inst_12 : AddCommMonoid M] →
                          [inst_13 : AddCommMonoid M₂] →
                            [inst_14 : Module R M] →
                              [inst_15 : Module R M₂] →
                                [inst_16 : Module S M] →
                                  [inst_17 : Module S M₂] →
                                    [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
              (map R S A B)) :
          (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
        (b • x) =
      HSMul.hSMul (β := Ω[B⁄S]) b
        ((LinearMap.liftBaseChange S
                ((↑R :
                    {S M : Type u_3} →
                      {M₂ : Type u_4} →
                        [inst : Semiring R] →
                          [inst_11 : Semiring S] →
                            [inst_12 : AddCommMonoid M] →
                              [inst_13 : AddCommMonoid M₂] →
                                [inst_14 : Module R M] →
                                  [inst_15 : Module R M₂] →
                                    [inst_16 : Module S M] →
                                      [inst_17 : Module S M₂] →
                                        [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
                  (map R S A B)) :
              (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
            x :
          Ω[B⁄S])),
  (LinearMap.liftBaseChange S
          ((↑R :
              {S M : Type u_3} →
                {M₂ : Type u_4} →
                  [inst : Semiring R] →
                    [inst_11 : Semiring S] →
                      [inst_12 : AddCommMonoid M] →
                        [inst_13 : AddCommMonoid M₂] →
                          [inst_14 : Module R M] →
                            [inst_15 : Module R M₂] →
                              [inst_16 : Module S M] →
                                [inst_17 : Module S M₂] →
                                  [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
            (map R S A B)) :
        (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
      ((s • b) • x) =
    HSMul.hSMul (β := Ω[B⁄S]) (s • b)
      ((LinearMap.liftBaseChange S
              ((↑R :
                  {S M : Type u_3} →
                    {M₂ : Type u_4} →
                      [inst : Semiring R] →
                        [inst_11 : Semiring S] →
                          [inst_12 : AddCommMonoid M] →
                            [inst_13 : AddCommMonoid M₂] →
                              [inst_14 : Module R M] →
                                [inst_15 : Module R M₂] →
                                  [inst_16 : Module S M] →
                                    [inst_17 : Module S M₂] →
                                      [LinearMap.CompatibleSMul M M₂ R S] → (fₗ : M →ₗ[S] M₂) → M →ₗ[R] M₂)
                (map R S A B)) :
            (a : S ⊗[R] Ω[A⁄R]) → Ω[B⁄S])
          x :
        Ω[B⁄S])
Tactic: induction b using h.1.inductionOn with
  | zero => simp only [zero_smul, map_zero]
  | smul s b e => rw [smul_assoc, map_smul, e, smul_assoc]
  | add b₁ b₂ e₁ e₂ => simp only [map_add, e₁, e₂, add_smul]
  | tmul a =>
    induction x
    · simp only [smul_zero, map_zero]
    · simp [smul_comm]
    · simp only [map_add, smul_add, *]
Cleared hypotheses: b