-- Mathlib/Geometry/Manifold/IsManifold/Basic.lean
theorem contDiffGroupoid_prod {I : ModelWithCorners 𝕜 E H} {I' : ModelWithCorners 𝕜 E' H'}
    {e : PartialHomeomorph H H} {e' : PartialHomeomorph H' H'} (he : e ∈ contDiffGroupoid n I)
    (he' : e' ∈ contDiffGroupoid n I') : e.prod e' ∈ contDiffGroupoid n (I.prod I')

Proof state:
case intro.intro.right
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
h3 :
  ContDiffOn 𝕜 n (Prod.map (↑I ∘ ↑e.symm ∘ ↑I.symm) (↑I' ∘ ↑e'.symm ∘ ↑I'.symm))
    ((fun p => (↑I p.1, ↑I' p.2)) '' e.target ×ˢ e'.target)
⊢ ContDiffOn 𝕜 n (↑(I.prod I') ∘ ↑(e.prod e').symm ∘ ↑(I.prod I').symm)
    (↑(I.prod I').symm ⁻¹' (e.prod e'.toPartialEquiv).target ∩ range ↑(I.prod I'))

Tactic:
rw [← (I.prod I').image_eq]

Result:
case intro.intro.right
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
h3 :
  ContDiffOn 𝕜 n (Prod.map (↑I ∘ ↑e.symm ∘ ↑I.symm) (↑I' ∘ ↑e'.symm ∘ ↑I'.symm))
    ((fun p => (↑I p.1, ↑I' p.2)) '' e.target ×ˢ e'.target)
⊢ ContDiffOn 𝕜 n (↑(I.prod I') ∘ ↑(e.prod e').symm ∘ ↑(I.prod I').symm)
    (↑(I.prod I') '' (e.prod e'.toPartialEquiv).target)

Proof state:
case intro.intro.left
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
h3 :
  ContDiffOn 𝕜 n (Prod.map (↑I ∘ ↑e ∘ ↑I.symm) (↑I' ∘ ↑e' ∘ ↑I'.symm))
    ((fun p => (↑I p.1, ↑I' p.2)) '' e.source ×ˢ e'.source)
⊢ ContDiffOn 𝕜 n (↑(I.prod I') ∘ ↑(e.prod e') ∘ ↑(I.prod I').symm)
    (↑(I.prod I').symm ⁻¹' e.source ×ˢ e'.source ∩ range ↑(I.prod I'))

Tactic:
rw [← (I.prod I').image_eq]

Result:
case intro.intro.left
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
h3 :
  ContDiffOn 𝕜 n (Prod.map (↑I ∘ ↑e ∘ ↑I.symm) (↑I' ∘ ↑e' ∘ ↑I'.symm))
    ((fun p => (↑I p.1, ↑I' p.2)) '' e.source ×ˢ e'.source)
⊢ ContDiffOn 𝕜 n (↑(I.prod I') ∘ ↑(e.prod e') ∘ ↑(I.prod I').symm) (↑(I.prod I') '' e.source ×ˢ e'.source)

Proof state:
case intro.intro
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
⊢ e.prod e' ∈ contDiffGroupoid n (I.prod I')

Tactic:
constructor <;> simp only [PartialEquiv.prod_source, PartialHomeomorph.prod_toPartialEquiv,
  contDiffPregroupoid]

Result:
case intro.intro.left
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
⊢ ContDiffOn 𝕜 n (↑(I.prod I') ∘ ↑(e.prod e') ∘ ↑(I.prod I').symm)
    (↑(I.prod I').symm ⁻¹' e.source ×ˢ e'.source ∩ range ↑(I.prod I'))

case intro.intro.right
n : WithTop ℕ∞
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
H : Type u_3
inst✝³ : TopologicalSpace H
E' : Type u_5
H' : Type u_6
inst✝² : NormedAddCommGroup E'
inst✝¹ : NormedSpace 𝕜 E'
inst✝ : TopologicalSpace H'
I : ModelWithCorners 𝕜 E H
I' : ModelWithCorners 𝕜 E' H'
e : PartialHomeomorph H H
e' : PartialHomeomorph H' H'
he : (contDiffPregroupoid n I).property (↑e) e.source
he_symm : (contDiffPregroupoid n I).property (↑e.symm) e.target
he' : (contDiffPregroupoid n I').property (↑e') e'.source
he'_symm : (contDiffPregroupoid n I').property (↑e'.symm) e'.target
⊢ ContDiffOn 𝕜 n (↑(I.prod I') ∘ ↑(e.prod e').symm ∘ ↑(I.prod I').symm)
    (↑(I.prod I').symm ⁻¹' (e.prod e'.toPartialEquiv).target ∩ range ↑(I.prod I'))