/-- Let `M` and `M'` be smooth manifolds with the same model-with-corners, `I`.  Then `f : M → M'`
is a local structomorphism for `I`, if and only if it is manifold-smooth on the domain of definition
in both directions. -/
theorem isLocalStructomorphOn_contDiffGroupoid_iff (f : PartialHomeomorph M M') :
    LiftPropOn (contDiffGroupoid ⊤ I).IsLocalStructomorphWithinAt f f.source ↔
      SmoothOn I I f f.source ∧ SmoothOn I I f.symm f.target := by
  constructor
  · intro h
    refine' ⟨isLocalStructomorphOn_contDiffGroupoid_iff_aux h,
      isLocalStructomorphOn_contDiffGroupoid_iff_aux _⟩
    -- todo: we can generalize this part of the proof to a lemma
    intro X hX
    let x := f.symm X
    have hx : x ∈ f.source := f.symm.mapsTo hX
    let c := chartAt H x
    let c' := chartAt H X
    obtain ⟨-, hxf⟩ := h x hx
    refine' ⟨(f.symm.continuousAt hX).continuousWithinAt, fun h2x => _⟩
    obtain ⟨e, he, h2e, hef, hex⟩ :
      ∃ e : PartialHomeomorph H H,
        e ∈ contDiffGroupoid ⊤ I ∧
          e.source ⊆ (c.symm ≫ₕ f ≫ₕ c').source ∧
            EqOn (c' ∘ f ∘ c.symm) e e.source ∧ c x ∈ e.source := by
      have h1 : c' = chartAt H (f x) := by simp only [f.right_inv hX]
      have h2 : c' ∘ f ∘ c.symm = ⇑(c.symm ≫ₕ f ≫ₕ c') := rfl
      have hcx : c x ∈ c.symm ⁻¹' f.source := by simp only [hx, mfld_simps]
      rw [h2]
      rw [← h1, h2, PartialHomeomorph.isLocalStructomorphWithinAt_iff'] at hxf
      · exact hxf hcx
      · mfld_set_tac
      · apply Or.inl
        simp only [hx, h1, mfld_simps]
    have h2X : c' X = e (c (f.symm X)) := by
      rw [← hef hex]
      dsimp only [Function.comp_def]
      

Proof state:
𝕜 : Type u_1
inst✝¹³ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝¹² : NormedAddCommGroup E
inst✝¹¹ : NormedSpace 𝕜 E
H : Type u_3
inst✝¹⁰ : TopologicalSpace H
I : ModelWithCorners 𝕜 E H
M : Type u_4
inst✝⁹ : TopologicalSpace M
inst✝⁸ : ChartedSpace H M
inst✝⁷ : SmoothManifoldWithCorners I M
E' : Type u_5
inst✝⁶ : NormedAddCommGroup E'
inst✝⁵ : NormedSpace 𝕜 E'
H' : Type u_6
inst✝⁴ : TopologicalSpace H'
I' : ModelWithCorners 𝕜 E' H'
M' : Type u_7
inst✝³ : TopologicalSpace M'
inst✝² : ChartedSpace H' M'
inst✝¹ : SmoothManifoldWithCorners I' M'
e✝ : PartialHomeomorph M H
x✝ : M
m n : ℕ∞
inst✝ : ChartedSpace H M'
IsM' : SmoothManifoldWithCorners I M'
f : PartialHomeomorph M M'
h : LiftPropOn (StructureGroupoid.IsLocalStructomorphWithinAt (contDiffGroupoid ⊤ I)) (↑f) f.source
X : M'
hX : X ∈ (PartialHomeomorph.symm f).toPartialEquiv.source
x : M := ↑(PartialHomeomorph.symm f) X
hx : x ∈ f.source
c : PartialHomeomorph M H := chartAt H x
c' : PartialHomeomorph M' H := chartAt H X
hxf :
  StructureGroupoid.IsLocalStructomorphWithinAt (contDiffGroupoid ⊤ I)
    (↑(chartAt H (↑f x)) ∘ ↑f ∘ ↑(PartialHomeomorph.symm (chartAt H x)))
    (↑(PartialHomeomorph.symm (chartAt H x)) ⁻¹' f.source) (↑(chartAt H x) x)
h2x : ↑(chartAt H X) X ∈ ↑(PartialHomeomorph.symm (chartAt H X)) ⁻¹' (PartialHomeomorph.symm f).toPartialEquiv.source
e : PartialHomeomorph H H
he : e ∈ contDiffGroupoid ⊤ I
h2e : e.source ⊆ (PartialHomeomorph.symm c ≫ₕ f ≫ₕ c').toPartialEquiv.source
hef : EqOn (↑c' ∘ ↑f ∘ ↑(PartialHomeomorph.symm c)) (↑e) e.source
hex : ↑c x ∈ e.source
⊢ ↑(chartAt H X) X =
    ↑(chartAt H X)
      (↑f
        (↑(PartialHomeomorph.symm (chartAt H (↑(PartialHomeomorph.symm f) X)))
          (↑(chartAt H (↑(PartialHomeomorph.symm f) X)) (↑(PartialHomeomorph.symm f) X))))

Next tactic:
have hfX : f.symm X ∈ c.source := by simp only [hX, mfld_simps]