-- Mathlib/Topology/Category/TopCat/Limits/Products.lean
theorem binaryCofan_isColimit_iff {X Y : TopCat} (c : BinaryCofan X Y) :
    Nonempty (IsColimit c) ↔
      IsOpenEmbedding c.inl ∧ IsOpenEmbedding c.inr ∧ IsCompl (range c.inl) (range c.inr)

Proof state:
case mpr.intro.intro.refine_1.refine_2
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
x : ↑c.1
⊢ ContinuousAt
    (fun x =>
      if h : x ∈ range ⇑(ConcreteCategory.hom c.inl) then
        (ConcreteCategory.hom f) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inl) ⋯).symm ⟨x, h⟩)
      else (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨x, ⋯⟩))
    x

Tactic:
by_cases h : x ∈ Set.range c.inl

Result:
case pos
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
x : ↑c.1
h : x ∈ range ⇑(ConcreteCategory.hom c.inl)
⊢ ContinuousAt
    (fun x =>
      if h : x ∈ range ⇑(ConcreteCategory.hom c.inl) then
        (ConcreteCategory.hom f) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inl) ⋯).symm ⟨x, h⟩)
      else (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨x, ⋯⟩))
    x

case neg
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
x : ↑c.1
h : x ∉ range ⇑(ConcreteCategory.hom c.inl)
⊢ ContinuousAt
    (fun x =>
      if h : x ∈ range ⇑(ConcreteCategory.hom c.inl) then
        (ConcreteCategory.hom f) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inl) ⋯).symm ⟨x, h⟩)
      else (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨x, ⋯⟩))
    x

Proof state:
case mpr.intro.intro.refine_1.refine_2
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
⊢ Continuous fun x =>
    if h : x ∈ range ⇑(ConcreteCategory.hom c.inl) then
      (ConcreteCategory.hom f) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inl) ⋯).symm ⟨x, h⟩)
    else (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨x, ⋯⟩)

Tactic:
rw [continuous_iff_continuousAt]

Result:
case mpr.intro.intro.refine_1.refine_2
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
⊢ ∀ (x : ↑c.1),
    ContinuousAt
      (fun x =>
        if h : x ∈ range ⇑(ConcreteCategory.hom c.inl) then
          (ConcreteCategory.hom f) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inl) ⋯).symm ⟨x, h⟩)
        else (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨x, ⋯⟩))
      x

Proof state:
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
⊢ ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ (range ⇑(ConcreteCategory.hom c.inl))ᶜ

Tactic:
exact fun _ => or_not

Result:
no goals

Proof state:
case pos
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
⊢ Continuous (⇑(ConcreteCategory.hom f) ∘ ⇑(Homeomorph.ofIsEmbedding ⇑(ConcreteCategory.hom c.inl) ⋯).symm)

Tactic:
continuity

Result:
no goals

Proof state:
case mpr.intro.intro.refine_3.w
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
x : ↑((pair X Y).obj { as := WalkingPair.right })
⊢ (if h : (Hom.hom c.inr) x ∈ range ⇑(ConcreteCategory.hom c.inl) then
      (ConcreteCategory.hom f) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inl) ⋯).symm ⟨(Hom.hom c.inr) x, h⟩)
    else (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨(Hom.hom c.inr) x, ⋯⟩)) =
    (ConcreteCategory.hom g) x

Tactic:
rw [dif_neg]

Result:
case mpr.intro.intro.refine_3.w
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
x : ↑((pair X Y).obj { as := WalkingPair.right })
⊢ (ConcreteCategory.hom g) ((Equiv.ofInjective ⇑(ConcreteCategory.hom c.inr) ⋯).symm ⟨(Hom.hom c.inr) x, ⋯⟩) =
    (ConcreteCategory.hom g) x

case mpr.intro.intro.refine_3.w.hnc
X Y : TopCat
c : BinaryCofan X Y
h₁ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inl)
h₂ : IsOpenEmbedding ⇑(ConcreteCategory.hom c.inr)
h₃ : IsCompl (range ⇑(ConcreteCategory.hom c.inl)) (range ⇑(ConcreteCategory.hom c.inr))
this :
  ∀ (x : ↑(((Functor.const (Discrete WalkingPair)).obj c.pt).obj { as := WalkingPair.left })),
    x ∈ range ⇑(ConcreteCategory.hom c.inl) ∨ x ∈ range ⇑(ConcreteCategory.hom c.inr)
T : TopCat
f : X ⟶ T
g : Y ⟶ T
x : ↑((pair X Y).obj { as := WalkingPair.right })
⊢ (Hom.hom c.inr) x ∉ range ⇑(ConcreteCategory.hom c.inl)