-- Lean 4 proof step from Mathlib.Geometry.Manifold.VectorField.LieBracket (goal)
Goal:
𝕜 : Type u_1
inst : NontriviallyNormedField 𝕜
H : Type u_2
inst_1 : TopologicalSpace H
E : Type u_3
inst_2 : NormedAddCommGroup E
inst_3 : NormedSpace 𝕜 E
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst_4 : TopologicalSpace M
inst_5 : ChartedSpace H M
H' : Type u_5
inst_6 : TopologicalSpace H'
E' : Type u_6
inst_7 : NormedAddCommGroup E'
inst_8 : NormedSpace 𝕜 E'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst_9 : TopologicalSpace M'
inst_10 : ChartedSpace H' M'
inst_11 : IsManifold I 2 M
inst_12 : IsManifold I' 2 M'
inst_13 : CompleteSpace E
f : M → M'
V W : (x : M') → TangentSpace I' x
x₀ : M
s : Set M
t : Set M'
hV : MDiffAt[t] (T% V) (f x₀)
hW : MDiffAt[t] (T% W) (f x₀)
hu : UniqueMDiffOn I s
hf : ContMDiffWithinAt I I' 2 f s x₀
hx₀ : x₀ ∈ s
hst : f ⁻¹' t ∈ 𝓝[s] x₀
hsymm :
  IsSymmSndFDerivWithinAt 𝕜 ((↑(extChartAt I' (f x₀)) : M' → E') ∘ f ∘ (↑(extChartAt I x₀).symm : E → M))
    ((↑(extChartAt I x₀).symm : E → M) ⁻¹' s ∩ range (↑I : H → E)) ((↑(extChartAt I x₀) : M → E) x₀)
A : (↑(extChartAt I x₀).symm : E → M) ((↑(extChartAt I x₀) : M → E) x₀) = x₀
hfi :
  ContinuousLinearMap.IsInvertible (M := TangentSpace I x₀) (M₂ := TangentSpace I' (f x₀)) (mfderivWithin I I' f s x₀)
this : CompleteSpace E'
u : Set M
u_open : IsOpen[inst_4] u
x₀u : x₀ ∈ u
u_mem : u ∈ 𝓝 x₀
s' : Set M := s ∩ u
ut : s' ⊆ f ⁻¹' t
maps_u : s' ⊆ f ⁻¹' (extChartAt I' (f x₀)).source
u_smooth : ContMDiffOn I I' 2 f s'
hs' : s' = s ∩ u
s'_eq : s' =ᶠ[𝓝 x₀] s
t' : Set M' := t ∩ (extChartAt I' (f x₀)).source
ht' : t' = t ∩ (extChartAt I' (f x₀)).source
⊢ Eq (α := TangentSpace I x₀) (mpullbackWithin I I' f (mlieBracketWithin I' V W t) s x₀)
    (mlieBracketWithin I (mpullbackWithin I I' f V s) (mpullbackWithin I I' f W s) s x₀)
have calc : Eq (α := TangentSpace I x₀) (mpullbackWithin I I' f (mlieBracketWithin I' V W t) s x₀)
  (mpullbackWithin I I' f (mlieBracketWithin I' V W t) s' x₀)
Tactic: calc mpullbackWithin I I' f (mlieBracketWithin I' V W t) s x₀
  _ = mpullbackWithin I I' f (mlieBracketWithin I' V W t) s' x₀ := by
    simp only [mpullbackWithin, hs', mfderivWithin_inter u_mem]
  _ = mpullbackWithin I I' f (mlieBracketWithin I' V W t') s' x₀ := by
    simp only [mpullbackWithin, ht', mlieBracketWithin_inter (extChartAt_source_mem_nhds (f x₀))]
  _ = mlieBracketWithin I (mpullbackWithin I I' f V s') (mpullbackWithin I I' f W s') s' x₀ := by
    apply mpullbackWithin_mlieBracketWithin_aux (t := t') (hV.mono inter_subset_left)
      (hW.mono inter_subset_left) (hu.inter u_open) u_smooth ⟨hx₀, x₀u⟩ inter_subset_right
      (fun y hy ↦ ⟨ut hy, maps_u hy⟩)
    apply hsymm.congr_set
    have : (extChartAt I x₀).symm ⁻¹' u ∈ 𝓝 (extChartAt I x₀ x₀) := by
      apply (continuousAt_extChartAt_symm x₀).preimage_mem_nhds
      apply u_open.mem_nhds (by simpa using x₀u)
    filter_upwards [this] with y hy
    change (y ∈ (extChartAt I x₀).symm ⁻¹' s ∩ range I) =
      (y ∈ (extChartAt I x₀).symm ⁻¹' (s ∩ u) ∩ range I)
    simp [-extChartAt, hy]
  _ = mlieBracketWithin I (mpullbackWithin I I' f V s') (mpullbackWithin I I' f W s') s x₀ := by
    simp only [hs', mlieBracketWithin_inter u_mem]
  _ = mlieBracketWithin I (mpullbackWithin I I' f V s) (mpullbackWithin I I' f W s) s x₀ := by
    apply Filter.EventuallyEq.mlieBracketWithin_vectorField_eq_of_mem _ _ hx₀
    · apply nhdsWithin_le_nhds
      filter_upwards [mfderivWithin_eventually_congr_set (I := I) (I' := I') (f := f) s'_eq]
        with y hy using by simp [mpullbackWithin, hy]
    · apply nhdsWithin_le_nhds
      filter_upwards [mfderivWithin_eventually_congr_set (I := I) (I' := I') (f := f) s'_eq]
        with y hy using by simp [mpullbackWithin, hy]