-- Lean 4 proof step from Mathlib.Geometry.Manifold.Algebra.LeftInvariantDerivation (hypothesis)
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)
    ((((𝒅ₕ (f := 𝑳 I g) ⋯ : (a : PointDerivation I 1) → PointDerivation I g)
              ((Derivation.evalAt 1 :
                  (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I 1)
                (↑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)
    ((((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)
⊢ Eq.{u_1 + 1} (α := 𝕜)
    ((((𝒅ₕ (f := 𝑳 I g) ⋯ : (a : PointDerivation I 1) → PointDerivation I g)
              ((Derivation.evalAt 1 :
                  (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I 1)
                ⁅(↑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)
    ((((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 hY : Eq.{u_1 + 1} (α := 𝕜)
  ((((𝒅ₕ (f := 𝑳 I g) sorry : (a : PointDerivation I 1) → PointDerivation I g)
            ((Derivation.evalAt 1 :
                (a : Derivation 𝕜 C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯ C^∞⟮I, G; 𝓘(𝕜, 𝕜), 𝕜⟯) → PointDerivation I 1)
              (↑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)
  ((((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)
Tactic: have hY := Derivation.congr_fun (left_invariant' g Y) (X f)