-- Lean 4 proof step from Mathlib.Geometry.Manifold.Algebra.LeftInvariantDerivation (goal)
Goal:
𝕜 : Type u_1
inst : NontriviallyNormedField 𝕜
n : ℕ∞ω
E : Type u_2
inst_1 : NormedAddCommGroup E
inst_2 : NormedSpace 𝕜 E
H : Type u_3
inst_3 : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
G : Type u_4
inst_4 : TopologicalSpace G
inst_5 : ChartedSpace H G
inst_6 : Monoid G
inst_7 : ContMDiffMul I ∞ G
g_1 h : G
r : 𝕜
X_1 Y_1 : LeftInvariantDerivation I G
f_1 f' : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯
X Y : LeftInvariantDerivation I G
g : G
f : C^∞⟮I, G; 𝕜⟯⟨g⟩
hX :
  Eq.{u_1 + 1} (α := 𝕜)
    (((((↑X : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
                (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)
              ((Y : (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) (ContMDiffMap.comp f (𝑳 I g))) :
            C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
          (a : G) → 𝕜)
        1 :
      𝕜)
    ((((Derivation.evalAt g : (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I g)
              (↑X : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
            PointDerivation I g) :
          (a : C^∞⟮I, G; 𝕜⟯⟨g⟩) → 𝕜)
        ((Y : (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) f : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
      𝕜)
hY :
  Eq.{u_1 + 1} (α := 𝕜)
    (((((↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
                (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)
              ((X : (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) (ContMDiffMap.comp f (𝑳 I g))) :
            C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
          (a : G) → 𝕜)
        1 :
      𝕜)
    ((((Derivation.evalAt g : (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I g)
              (↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
            PointDerivation I g) :
          (a : C^∞⟮I, G; 𝕜⟯⟨g⟩) → 𝕜)
        ((X : (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) f : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
      𝕜)
⊢ Eq.{u_1 + 1} (α := 𝕜)
    ((((⁅(↑X : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯),
                  (↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)⁆ :
                (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)
              (ContMDiffMap.comp f (𝑳 I g)) :
            C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
          (a : G) → 𝕜)
        1 :
      𝕜)
    ((((Derivation.evalAt g : (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I g)
              ⁅(↑X : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯),
                (↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)⁆ :
            PointDerivation I g) :
          (a : C^∞⟮I, G; 𝕜⟯⟨g⟩) → 𝕜)
        f :
      𝕜)
have H₁ : HSub.hSub (α := 𝕜) (β := 𝕜)
    ((((X : (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)
              (((↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
                  (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)
                (ContMDiffMap.comp f (𝑳 I g))) :
            C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
          (a : G) → 𝕜)
        1 :
      𝕜)
    (((((↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
                (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)
              ((X : (a : C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) (ContMDiffMap.comp f (𝑳 I g))) :
            C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) :
          (a : G) → 𝕜)
        1 :
      𝕜) =
  (((Derivation.evalAt g : (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I g)
          ⁅(↑X : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯),
            (↑Y : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯)⁆ :
        PointDerivation I g) :
      (a : C^∞⟮I, G; 𝕜⟯⟨g⟩) → 𝕜)
    f
Tactic: rw [Derivation.commutator_apply, ContMDiffMap.coe_sub, Pi.sub_apply, coe_derivation]