-- Mathlib/Topology/FiberBundle/Constructions.lean
theorem Prod.continuous_to_fun : ContinuousOn (Prod.toFun' e₁ e₂)
    (π (F₁ × F₂) (E₁ ×ᵇ E₂) ⁻¹' (e₁.baseSet ∩ e₂.baseSet))

Proof state:
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
⊢ ContinuousOn (toFun' e₁ e₂) (TotalSpace.proj ⁻¹' (e₁.baseSet ∩ e₂.baseSet))

Tactic:
let f₁ : TotalSpace (F₁ × F₂) (E₁ ×ᵇ E₂) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p ↦ ((⟨p.1, p.2.1⟩ : TotalSpace F₁ E₁), (⟨p.1, p.2.2⟩ : TotalSpace F₂ E₂))

Result:
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
⊢ ContinuousOn (toFun' e₁ e₂) (TotalSpace.proj ⁻¹' (e₁.baseSet ∩ e₂.baseSet))

Proof state:
case refine_2.mk.mk.intro
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
f₂ : TotalSpace F₁ E₁ × TotalSpace F₂ E₂ → (B × F₁) × B × F₂ := fun p => (↑e₁ p.1, ↑e₂ p.2)
f₃ : (B × F₁) × B × F₂ → B × F₁ × F₂ := fun p => (p.1.1, p.1.2, p.2.2)
hf₁ : Continuous f₁
hf₂ : ContinuousOn f₂ (e₁.source ×ˢ e₂.source)
hf₃ : Continuous f₃
b : B
v₁ : E₁ b
v₂ : E₂ b
hb₁ : { proj := b, snd := (v₁, v₂) }.proj ∈ e₁.baseSet
right✝ : { proj := b, snd := (v₁, v₂) }.proj ∈ e₂.baseSet
⊢ b = (↑e₁ { proj := b, snd := v₁ }).1

Tactic:
rw [e₁.coe_fst]

Result:
case refine_2.mk.mk.intro
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
f₂ : TotalSpace F₁ E₁ × TotalSpace F₂ E₂ → (B × F₁) × B × F₂ := fun p => (↑e₁ p.1, ↑e₂ p.2)
f₃ : (B × F₁) × B × F₂ → B × F₁ × F₂ := fun p => (p.1.1, p.1.2, p.2.2)
hf₁ : Continuous f₁
hf₂ : ContinuousOn f₂ (e₁.source ×ˢ e₂.source)
hf₃ : Continuous f₃
b : B
v₁ : E₁ b
v₂ : E₂ b
hb₁ : { proj := b, snd := (v₁, v₂) }.proj ∈ e₁.baseSet
right✝ : { proj := b, snd := (v₁, v₂) }.proj ∈ e₂.baseSet
⊢ { proj := b, snd := v₁ } ∈ e₁.source

Proof state:
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
f₂ : TotalSpace F₁ E₁ × TotalSpace F₂ E₂ → (B × F₁) × B × F₂ := fun p => (↑e₁ p.1, ↑e₂ p.2)
f₃ : (B × F₁) × B × F₂ → B × F₁ × F₂ := fun p => (p.1.1, p.1.2, p.2.2)
⊢ ContinuousOn (toFun' e₁ e₂) (TotalSpace.proj ⁻¹' (e₁.baseSet ∩ e₂.baseSet))

Tactic:
have hf₁ : Continuous f₁ := (Prod.isInducing_diag F₁ E₁ F₂ E₂).continuous

Result:
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
f₂ : TotalSpace F₁ E₁ × TotalSpace F₂ E₂ → (B × F₁) × B × F₂ := fun p => (↑e₁ p.1, ↑e₂ p.2)
f₃ : (B × F₁) × B × F₂ → B × F₁ × F₂ := fun p => (p.1.1, p.1.2, p.2.2)
hf₁ : Continuous f₁
⊢ ContinuousOn (toFun' e₁ e₂) (TotalSpace.proj ⁻¹' (e₁.baseSet ∩ e₂.baseSet))

Proof state:
case refine_1
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
f₂ : TotalSpace F₁ E₁ × TotalSpace F₂ E₂ → (B × F₁) × B × F₂ := fun p => (↑e₁ p.1, ↑e₂ p.2)
f₃ : (B × F₁) × B × F₂ → B × F₁ × F₂ := fun p => (p.1.1, p.1.2, p.2.2)
hf₁ : Continuous f₁
hf₂ : ContinuousOn f₂ (e₁.source ×ˢ e₂.source)
hf₃ : Continuous f₃
⊢ MapsTo f₁ (TotalSpace.proj ⁻¹' (e₁.baseSet ∩ e₂.baseSet)) (e₁.source ×ˢ e₂.source)

Tactic:
rw [e₁.source_eq, e₂.source_eq]

Result:
case refine_1
B : Type u_1
inst✝⁴ : TopologicalSpace B
F₁ : Type u_2
inst✝³ : TopologicalSpace F₁
E₁ : B → Type u_3
inst✝² : TopologicalSpace (TotalSpace F₁ E₁)
F₂ : Type u_4
inst✝¹ : TopologicalSpace F₂
E₂ : B → Type u_5
inst✝ : TopologicalSpace (TotalSpace F₂ E₂)
e₁ : Trivialization F₁ TotalSpace.proj
e₂ : Trivialization F₂ TotalSpace.proj
f₁ : (TotalSpace (F₁ × F₂) fun x => E₁ x × E₂ x) → TotalSpace F₁ E₁ × TotalSpace F₂ E₂ :=
  fun p => ({ proj := p.proj, snd := p.snd.1 }, { proj := p.proj, snd := p.snd.2 })
f₂ : TotalSpace F₁ E₁ × TotalSpace F₂ E₂ → (B × F₁) × B × F₂ := fun p => (↑e₁ p.1, ↑e₂ p.2)
f₃ : (B × F₁) × B × F₂ → B × F₁ × F₂ := fun p => (p.1.1, p.1.2, p.2.2)
hf₁ : Continuous f₁
hf₂ : ContinuousOn f₂ (e₁.source ×ˢ e₂.source)
hf₃ : Continuous f₃
⊢ MapsTo f₁ (TotalSpace.proj ⁻¹' (e₁.baseSet ∩ e₂.baseSet))
    ((TotalSpace.proj ⁻¹' e₁.baseSet) ×ˢ (TotalSpace.proj ⁻¹' e₂.baseSet))