/-- If `s` has the unique differential property at `x`, `f` is differentiable within `s` at x` and
its derivative has Dense range, then `f '' s` has the Unique differential property at `f x`. -/
theorem UniqueMDiffWithinAt.image_denseRange (hs : UniqueMDiffWithinAt I s x)
    {f : M → M'} {f' : E →L[𝕜] E'} (hf : HasMFDerivWithinAt I I' f s x f')
    (hd : DenseRange f') : UniqueMDiffWithinAt I' (f '' s) (f x) := by
  /- Rewrite in coordinates, apply `HasFDerivWithinAt.uniqueDiffWithinAt`. -/
  have := hs.inter' <| hf.1 (extChartAt_source_mem_nhds I' (f x))
  refine (((hf.2.mono ?sub1).uniqueDiffWithinAt this hd).mono ?sub2).congr_pt ?pt
  

Proof state:
case sub1
𝕜 : Type u_1
inst✝¹² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹¹ : NormedAddCommGroup E
inst✝¹⁰ : NormedSpace 𝕜 E
H : Type u_3
inst✝⁹ : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝⁸ : TopologicalSpace M
inst✝⁷ : ChartedSpace H M
inst✝⁶ : SmoothManifoldWithCorners I M
E' : Type u_5
inst✝⁵ : NormedAddCommGroup E'
inst✝⁴ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝³ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝² : TopologicalSpace M'
inst✝¹ : ChartedSpace H' M'
inst✝ : SmoothManifoldWithCorners I' M'
s : Set M
x : M
hs : UniqueMDiffWithinAt I s x
f : M → M'
f' : E →L[𝕜] E'
hf : HasMFDerivWithinAt I I' f s x f'
hd : DenseRange ⇑f'
this : UniqueMDiffWithinAt I (s ∩ f ⁻¹' (extChartAt I' (f x)).source) x
⊢ ↑(PartialEquiv.symm (extChartAt I x)) ⁻¹' (s ∩ f ⁻¹' (extChartAt I' (f x)).source) ∩ range ↑I ⊆
    ↑(PartialEquiv.symm (extChartAt I x)) ⁻¹' s ∩ range ↑I
case sub2
𝕜 : Type u_1
inst✝¹² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹¹ : NormedAddCommGroup E
inst✝¹⁰ : NormedSpace 𝕜 E
H : Type u_3
inst✝⁹ : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝⁸ : TopologicalSpace M
inst✝⁷ : ChartedSpace H M
inst✝⁶ : SmoothManifoldWithCorners I M
E' : Type u_5
inst✝⁵ : NormedAddCommGroup E'
inst✝⁴ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝³ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝² : TopologicalSpace M'
inst✝¹ : ChartedSpace H' M'
inst✝ : SmoothManifoldWithCorners I' M'
s : Set M
x : M
hs : UniqueMDiffWithinAt I s x
f : M → M'
f' : E →L[𝕜] E'
hf : HasMFDerivWithinAt I I' f s x f'
hd : DenseRange ⇑f'
this : UniqueMDiffWithinAt I (s ∩ f ⁻¹' (extChartAt I' (f x)).source) x
⊢ writtenInExtChartAt I I' x f ''
      (↑(PartialEquiv.symm (extChartAt I x)) ⁻¹' (s ∩ f ⁻¹' (extChartAt I' (f x)).source) ∩ range ↑I) ⊆
    ↑(PartialEquiv.symm (extChartAt I' (f x))) ⁻¹' (f '' s) ∩ range ↑I'
case pt
𝕜 : Type u_1
inst✝¹² : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹¹ : NormedAddCommGroup E
inst✝¹⁰ : NormedSpace 𝕜 E
H : Type u_3
inst✝⁹ : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝⁸ : TopologicalSpace M
inst✝⁷ : ChartedSpace H M
inst✝⁶ : SmoothManifoldWithCorners I M
E' : Type u_5
inst✝⁵ : NormedAddCommGroup E'
inst✝⁴ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝³ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝² : TopologicalSpace M'
inst✝¹ : ChartedSpace H' M'
inst✝ : SmoothManifoldWithCorners I' M'
s : Set M
x : M
hs : UniqueMDiffWithinAt I s x
f : M → M'
f' : E →L[𝕜] E'
hf : HasMFDerivWithinAt I I' f s x f'
hd : DenseRange ⇑f'
this : UniqueMDiffWithinAt I (s ∩ f ⁻¹' (extChartAt I' (f x)).source) x
⊢ writtenInExtChartAt I I' x f (↑(extChartAt I x) x) = ↑(extChartAt I' (f x)) (f x)

Next tactic:
case pt => simp only [mfld_simps]