-- Mathlib/Geometry/Manifold/IntegralCurve/ExistUnique.lean
theorem isIntegralCurveAt_eventuallyEq_of_contMDiffAt (hγt₀ : I.IsInteriorPoint (γ t₀))
    (hv : ContMDiffAt I I.tangent 1 (fun x ↦ (⟨x, v x⟩ : TangentBundle I M)) (γ t₀))
    (hγ : IsIntegralCurveAt γ v t₀) (hγ' : IsIntegralCurveAt γ' v t₀) (h : γ t₀ = γ' t₀) :
    γ =ᶠ[𝓝 t₀] γ'

Proof state:
case intro.intro.intro.intro
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
H : Type u_2
inst✝³ : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type u_3
inst✝² : TopologicalSpace M
inst✝¹ : ChartedSpace H M
inst✝ : IsManifold I 1 M
γ γ' : ℝ → M
v : (x : M) → TangentSpace I x
t₀ : ℝ
hγt₀ : I.IsInteriorPoint (γ t₀)
hγ : IsIntegralCurveAt γ v t₀
hγ' : IsIntegralCurveAt γ' v t₀
h : γ t₀ = γ' t₀
v' : E → E :=
  fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
hv' :
  v' = fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
left✝ : ContinuousAt (fun x => { proj := x, snd := v x }) (γ t₀)
hv :
  ContDiffWithinAt ℝ 1
    (↑(extChartAt I.tangent { proj := γ t₀, snd := v (γ t₀) }) ∘
      (fun x => { proj := x, snd := v x }) ∘ ↑(extChartAt I (γ t₀)).symm)
    (range ↑I) (↑(extChartAt I (γ t₀)) (γ t₀))
K : NNReal
s : Set E
hs : s ∈ 𝓝 (↑(extChartAt I (γ t₀)) (γ t₀))
hlip✝ : LipschitzOnWith K v' s
hlip : ∀ (t : ℝ), LipschitzOnWith K ((fun x => v') t) ((fun x => s) t)
hsrc : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → ∀ᶠ (t : ℝ) in 𝓝 t₀, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t
hmem : ∀ {g : ℝ → M} {t : ℝ}, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t → g t ∈ (extChartAt I (g t₀)).source
hdrv :
  ∀ {g : ℝ → M},
    IsIntegralCurveAt g v t₀ →
      γ t₀ = g t₀ →
        ∀ᶠ (t : ℝ) in 𝓝 t₀,
          HasDerivAt (↑(extChartAt I (g t₀)) ∘ g) ((fun x => v') t ((↑(extChartAt I (g t₀)) ∘ g) t)) t ∧
            (↑(extChartAt I (g t₀)) ∘ g) t ∈ (fun x => s) t
⊢ γ =ᶠ[𝓝 t₀] γ'

Tactic:
have heq {g} (hg : IsIntegralCurveAt g v t₀) :
  g =ᶠ[𝓝 t₀] (extChartAt I (g t₀)).symm ∘ ↑(extChartAt I (g t₀)) ∘ g := by
  apply (hsrc hg).mono
  intros t ht
  rw [Function.comp_apply, Function.comp_apply, PartialEquiv.left_inv _ (hmem ht)]

Result:
case intro.intro.intro.intro
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
H : Type u_2
inst✝³ : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type u_3
inst✝² : TopologicalSpace M
inst✝¹ : ChartedSpace H M
inst✝ : IsManifold I 1 M
γ γ' : ℝ → M
v : (x : M) → TangentSpace I x
t₀ : ℝ
hγt₀ : I.IsInteriorPoint (γ t₀)
hγ : IsIntegralCurveAt γ v t₀
hγ' : IsIntegralCurveAt γ' v t₀
h : γ t₀ = γ' t₀
v' : E → E :=
  fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
hv' :
  v' = fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
left✝ : ContinuousAt (fun x => { proj := x, snd := v x }) (γ t₀)
hv :
  ContDiffWithinAt ℝ 1
    (↑(extChartAt I.tangent { proj := γ t₀, snd := v (γ t₀) }) ∘
      (fun x => { proj := x, snd := v x }) ∘ ↑(extChartAt I (γ t₀)).symm)
    (range ↑I) (↑(extChartAt I (γ t₀)) (γ t₀))
K : NNReal
s : Set E
hs : s ∈ 𝓝 (↑(extChartAt I (γ t₀)) (γ t₀))
hlip✝ : LipschitzOnWith K v' s
hlip : ∀ (t : ℝ), LipschitzOnWith K ((fun x => v') t) ((fun x => s) t)
hsrc : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → ∀ᶠ (t : ℝ) in 𝓝 t₀, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t
hmem : ∀ {g : ℝ → M} {t : ℝ}, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t → g t ∈ (extChartAt I (g t₀)).source
hdrv :
  ∀ {g : ℝ → M},
    IsIntegralCurveAt g v t₀ →
      γ t₀ = g t₀ →
        ∀ᶠ (t : ℝ) in 𝓝 t₀,
          HasDerivAt (↑(extChartAt I (g t₀)) ∘ g) ((fun x => v') t ((↑(extChartAt I (g t₀)) ∘ g) t)) t ∧
            (↑(extChartAt I (g t₀)) ∘ g) t ∈ (fun x => s) t
heq : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → g =ᶠ[𝓝 t₀] ↑(extChartAt I (g t₀)).symm ∘ ↑(extChartAt I (g t₀)) ∘ g
⊢ γ =ᶠ[𝓝 t₀] γ'

Proof state:
case intro.intro.intro.intro
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
H : Type u_2
inst✝³ : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type u_3
inst✝² : TopologicalSpace M
inst✝¹ : ChartedSpace H M
inst✝ : IsManifold I 1 M
γ γ' : ℝ → M
v : (x : M) → TangentSpace I x
t₀ : ℝ
hγt₀ : I.IsInteriorPoint (γ t₀)
hγ : IsIntegralCurveAt γ v t₀
hγ' : IsIntegralCurveAt γ' v t₀
h : γ t₀ = γ' t₀
v' : E → E :=
  fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
hv' :
  v' = fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
left✝ : ContinuousAt (fun x => { proj := x, snd := v x }) (γ t₀)
hv :
  ContDiffWithinAt ℝ 1
    (↑(extChartAt I.tangent { proj := γ t₀, snd := v (γ t₀) }) ∘
      (fun x => { proj := x, snd := v x }) ∘ ↑(extChartAt I (γ t₀)).symm)
    (range ↑I) (↑(extChartAt I (γ t₀)) (γ t₀))
K : NNReal
s : Set E
hs : s ∈ 𝓝 (↑(extChartAt I (γ t₀)) (γ t₀))
hlip✝ : LipschitzOnWith K v' s
hlip : ∀ (t : ℝ), LipschitzOnWith K ((fun x => v') t) ((fun x => s) t)
hsrc : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → ∀ᶠ (t : ℝ) in 𝓝 t₀, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t
hmem : ∀ {g : ℝ → M} {t : ℝ}, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t → g t ∈ (extChartAt I (g t₀)).source
hdrv :
  ∀ {g : ℝ → M},
    IsIntegralCurveAt g v t₀ →
      γ t₀ = g t₀ →
        ∀ᶠ (t : ℝ) in 𝓝 t₀,
          HasDerivAt (↑(extChartAt I (g t₀)) ∘ g) ((fun x => v') t ((↑(extChartAt I (g t₀)) ∘ g) t)) t ∧
            (↑(extChartAt I (g t₀)) ∘ g) t ∈ (fun x => s) t
heq : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → g =ᶠ[𝓝 t₀] ↑(extChartAt I (g t₀)).symm ∘ ↑(extChartAt I (g t₀)) ∘ g
⊢ γ =ᶠ[𝓝 t₀] γ'

Tactic:
suffices (extChartAt I (γ t₀)) ∘ γ =ᶠ[𝓝 t₀] (extChartAt I (γ' t₀)) ∘ γ' from
  (heq hγ).trans <| (this.fun_comp (extChartAt I (γ t₀)).symm).trans (h ▸ (heq hγ').symm)

Result:
case intro.intro.intro.intro
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
H : Type u_2
inst✝³ : TopologicalSpace H
I : ModelWithCorners ℝ E H
M : Type u_3
inst✝² : TopologicalSpace M
inst✝¹ : ChartedSpace H M
inst✝ : IsManifold I 1 M
γ γ' : ℝ → M
v : (x : M) → TangentSpace I x
t₀ : ℝ
hγt₀ : I.IsInteriorPoint (γ t₀)
hγ : IsIntegralCurveAt γ v t₀
hγ' : IsIntegralCurveAt γ' v t₀
h : γ t₀ = γ' t₀
v' : E → E :=
  fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
hv' :
  v' = fun x =>
    (tangentCoordChange I (↑(extChartAt I (γ t₀)).symm x) (γ t₀) (↑(extChartAt I (γ t₀)).symm x))
      (v (↑(extChartAt I (γ t₀)).symm x))
left✝ : ContinuousAt (fun x => { proj := x, snd := v x }) (γ t₀)
hv :
  ContDiffWithinAt ℝ 1
    (↑(extChartAt I.tangent { proj := γ t₀, snd := v (γ t₀) }) ∘
      (fun x => { proj := x, snd := v x }) ∘ ↑(extChartAt I (γ t₀)).symm)
    (range ↑I) (↑(extChartAt I (γ t₀)) (γ t₀))
K : NNReal
s : Set E
hs : s ∈ 𝓝 (↑(extChartAt I (γ t₀)) (γ t₀))
hlip✝ : LipschitzOnWith K v' s
hlip : ∀ (t : ℝ), LipschitzOnWith K ((fun x => v') t) ((fun x => s) t)
hsrc : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → ∀ᶠ (t : ℝ) in 𝓝 t₀, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t
hmem : ∀ {g : ℝ → M} {t : ℝ}, g ⁻¹' (extChartAt I (g t₀)).source ∈ 𝓝 t → g t ∈ (extChartAt I (g t₀)).source
hdrv :
  ∀ {g : ℝ → M},
    IsIntegralCurveAt g v t₀ →
      γ t₀ = g t₀ →
        ∀ᶠ (t : ℝ) in 𝓝 t₀,
          HasDerivAt (↑(extChartAt I (g t₀)) ∘ g) ((fun x => v') t ((↑(extChartAt I (g t₀)) ∘ g) t)) t ∧
            (↑(extChartAt I (g t₀)) ∘ g) t ∈ (fun x => s) t
heq : ∀ {g : ℝ → M}, IsIntegralCurveAt g v t₀ → g =ᶠ[𝓝 t₀] ↑(extChartAt I (g t₀)).symm ∘ ↑(extChartAt I (g t₀)) ∘ g
⊢ ↑(extChartAt I (γ t₀)) ∘ γ =ᶠ[𝓝 t₀] ↑(extChartAt I (γ' t₀)) ∘ γ'