-- Mathlib/Geometry/Manifold/ContMDiff/Basic.lean
theorem ContMDiffWithinAt.comp {t : Set M'} {g : M' → M''} (x : M)
    (hg : ContMDiffWithinAt I' I'' n g t (f x)) (hf : ContMDiffWithinAt I I' n f s x)
    (st : MapsTo f s t) : ContMDiffWithinAt I I'' n (g ∘ f) s x

Proof state:
𝕜 : Type u_1
inst✝¹⁵ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹⁴ : NormedAddCommGroup E
inst✝¹³ : NormedSpace 𝕜 E
H : Type u_3
inst✝¹² : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝¹¹ : TopologicalSpace M
E' : Type u_5
inst✝¹⁰ : NormedAddCommGroup E'
inst✝⁹ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝⁸ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝⁷ : TopologicalSpace M'
E'' : Type u_8
inst✝⁶ : NormedAddCommGroup E''
inst✝⁵ : NormedSpace 𝕜 E''
H'' : Type u_9
inst✝⁴ : TopologicalSpace H''
I'' : ModelWithCorners 𝕜 E'' H''
M'' : Type u_10
inst✝³ : TopologicalSpace M''
inst✝² : ChartedSpace H M
inst✝¹ : ChartedSpace H' M'
inst✝ : ChartedSpace H'' M''
f : M → M'
s : Set M
n : WithTop ℕ∞
t : Set M'
g : M' → M''
x : M
st : MapsTo f s t
e : PartialEquiv M E := extChartAt I x
e' : PartialEquiv M' E' := extChartAt I' (f x)
hg :
  ContinuousWithinAt g t (f x) ∧
    ContDiffWithinAt 𝕜 n (↑(extChartAt I'' (g (f x))) ∘ g ∘ ↑e'.symm) (↑e'.symm ⁻¹' t ∩ range ↑I')
      (writtenInExtChartAt I I' x f (↑e x))
hf : ContinuousWithinAt f s x ∧ ContDiffWithinAt 𝕜 n (↑e' ∘ f ∘ ↑e.symm) (↑e.symm ⁻¹' s ∩ range ↑I) (↑e x)
this : ↑e' (f x) = writtenInExtChartAt I I' x f (↑e x)
⊢ ContDiffWithinAt 𝕜 n (↑(extChartAt I'' ((g ∘ f) x)) ∘ (g ∘ f) ∘ ↑e.symm) (↑e.symm ⁻¹' s ∩ range ↑I) (↑e x)

Tactic:
have A : ∀ᶠ y in 𝓝[e.symm ⁻¹' s ∩ range I] e x, f (e.symm y) ∈ t ∧ f (e.symm y) ∈ e'.source := by
  simp only [e, ← map_extChartAt_nhdsWithin, eventually_map]
  filter_upwards [hf.1.tendsto (extChartAt_source_mem_nhds (I := I') (f x)),
    inter_mem_nhdsWithin s (extChartAt_source_mem_nhds (I := I) x)]
  rintro x' (hfx' : f x' ∈ e'.source) ⟨hx's, hx'⟩
  simp only [e, e.map_source hx', true_and, e.left_inv hx', st hx's, *]

Result:
𝕜 : Type u_1
inst✝¹⁵ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹⁴ : NormedAddCommGroup E
inst✝¹³ : NormedSpace 𝕜 E
H : Type u_3
inst✝¹² : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝¹¹ : TopologicalSpace M
E' : Type u_5
inst✝¹⁰ : NormedAddCommGroup E'
inst✝⁹ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝⁸ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝⁷ : TopologicalSpace M'
E'' : Type u_8
inst✝⁶ : NormedAddCommGroup E''
inst✝⁵ : NormedSpace 𝕜 E''
H'' : Type u_9
inst✝⁴ : TopologicalSpace H''
I'' : ModelWithCorners 𝕜 E'' H''
M'' : Type u_10
inst✝³ : TopologicalSpace M''
inst✝² : ChartedSpace H M
inst✝¹ : ChartedSpace H' M'
inst✝ : ChartedSpace H'' M''
f : M → M'
s : Set M
n : WithTop ℕ∞
t : Set M'
g : M' → M''
x : M
st : MapsTo f s t
e : PartialEquiv M E := extChartAt I x
e' : PartialEquiv M' E' := extChartAt I' (f x)
hg :
  ContinuousWithinAt g t (f x) ∧
    ContDiffWithinAt 𝕜 n (↑(extChartAt I'' (g (f x))) ∘ g ∘ ↑e'.symm) (↑e'.symm ⁻¹' t ∩ range ↑I')
      (writtenInExtChartAt I I' x f (↑e x))
hf : ContinuousWithinAt f s x ∧ ContDiffWithinAt 𝕜 n (↑e' ∘ f ∘ ↑e.symm) (↑e.symm ⁻¹' s ∩ range ↑I) (↑e x)
this : ↑e' (f x) = writtenInExtChartAt I I' x f (↑e x)
A : ∀ᶠ (y : E) in 𝓝[↑e.symm ⁻¹' s ∩ range ↑I] ↑e x, f (↑e.symm y) ∈ t ∧ f (↑e.symm y) ∈ e'.source
⊢ ContDiffWithinAt 𝕜 n (↑(extChartAt I'' ((g ∘ f) x)) ∘ (g ∘ f) ∘ ↑e.symm) (↑e.symm ⁻¹' s ∩ range ↑I) (↑e x)

Proof state:
case h
𝕜 : Type u_1
inst✝¹⁵ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹⁴ : NormedAddCommGroup E
inst✝¹³ : NormedSpace 𝕜 E
H : Type u_3
inst✝¹² : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝¹¹ : TopologicalSpace M
E' : Type u_5
inst✝¹⁰ : NormedAddCommGroup E'
inst✝⁹ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝⁸ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝⁷ : TopologicalSpace M'
E'' : Type u_8
inst✝⁶ : NormedAddCommGroup E''
inst✝⁵ : NormedSpace 𝕜 E''
H'' : Type u_9
inst✝⁴ : TopologicalSpace H''
I'' : ModelWithCorners 𝕜 E'' H''
M'' : Type u_10
inst✝³ : TopologicalSpace M''
inst✝² : ChartedSpace H M
inst✝¹ : ChartedSpace H' M'
inst✝ : ChartedSpace H'' M''
f : M → M'
s : Set M
n : WithTop ℕ∞
t : Set M'
g : M' → M''
x : M
st : MapsTo f s t
e : PartialEquiv M E := extChartAt I x
e' : PartialEquiv M' E' := extChartAt I' (f x)
hg :
  ContinuousWithinAt g t (f x) ∧
    ContDiffWithinAt 𝕜 n (↑(extChartAt I'' (g (f x))) ∘ g ∘ ↑e'.symm) (↑e'.symm ⁻¹' t ∩ range ↑I')
      (writtenInExtChartAt I I' x f (↑e x))
hf : ContinuousWithinAt f s x ∧ ContDiffWithinAt 𝕜 n (↑e' ∘ f ∘ ↑e.symm) (↑e.symm ⁻¹' s ∩ range ↑I) (↑e x)
this : ↑e' (f x) = writtenInExtChartAt I I' x f (↑e x)
A : ∀ᶠ (y : E) in 𝓝[↑e.symm ⁻¹' s ∩ range ↑I] ↑e x, f (↑e.symm y) ∈ t ∧ f (↑e.symm y) ∈ e'.source
⊢ ∀ (a : E),
    f (↑e.symm a) ∈ t ∧ f (↑e.symm a) ∈ e'.source →
      (↑(extChartAt I'' ((g ∘ f) x)) ∘ (g ∘ f) ∘ ↑e.symm) a =
        ((↑(extChartAt I'' (g (f x))) ∘ g ∘ ↑e'.symm) ∘ writtenInExtChartAt I I' x f) a

Tactic:
rintro x' ⟨-, hfx'⟩

Result:
case h.intro
𝕜 : Type u_1
inst✝¹⁵ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹⁴ : NormedAddCommGroup E
inst✝¹³ : NormedSpace 𝕜 E
H : Type u_3
inst✝¹² : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝¹¹ : TopologicalSpace M
E' : Type u_5
inst✝¹⁰ : NormedAddCommGroup E'
inst✝⁹ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝⁸ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝⁷ : TopologicalSpace M'
E'' : Type u_8
inst✝⁶ : NormedAddCommGroup E''
inst✝⁵ : NormedSpace 𝕜 E''
H'' : Type u_9
inst✝⁴ : TopologicalSpace H''
I'' : ModelWithCorners 𝕜 E'' H''
M'' : Type u_10
inst✝³ : TopologicalSpace M''
inst✝² : ChartedSpace H M
inst✝¹ : ChartedSpace H' M'
inst✝ : ChartedSpace H'' M''
f : M → M'
s : Set M
n : WithTop ℕ∞
t : Set M'
g : M' → M''
x : M
st : MapsTo f s t
e : PartialEquiv M E := extChartAt I x
e' : PartialEquiv M' E' := extChartAt I' (f x)
hg :
  ContinuousWithinAt g t (f x) ∧
    ContDiffWithinAt 𝕜 n (↑(extChartAt I'' (g (f x))) ∘ g ∘ ↑e'.symm) (↑e'.symm ⁻¹' t ∩ range ↑I')
      (writtenInExtChartAt I I' x f (↑e x))
hf : ContinuousWithinAt f s x ∧ ContDiffWithinAt 𝕜 n (↑e' ∘ f ∘ ↑e.symm) (↑e.symm ⁻¹' s ∩ range ↑I) (↑e x)
this : ↑e' (f x) = writtenInExtChartAt I I' x f (↑e x)
A : ∀ᶠ (y : E) in 𝓝[↑e.symm ⁻¹' s ∩ range ↑I] ↑e x, f (↑e.symm y) ∈ t ∧ f (↑e.symm y) ∈ e'.source
x' : E
hfx' : f (↑e.symm x') ∈ e'.source
⊢ (↑(extChartAt I'' ((g ∘ f) x)) ∘ (g ∘ f) ∘ ↑e.symm) x' =
    ((↑(extChartAt I'' (g (f x))) ∘ g ∘ ↑e'.symm) ∘ writtenInExtChartAt I I' x f) x'