-- Mathlib/CategoryTheory/Limits/Shapes/KernelPair.lean
theorem pullback {X Y Z A : C} {g : Y ⟶ Z} {a₁ a₂ : A ⟶ Y} (h : IsKernelPair g a₁ a₂)
    (f : X ⟶ Z) [HasPullback f g] [HasPullback f (a₁ ≫ g)] :
    IsKernelPair (pullback.fst f g)
      (pullback.map f _ f _ (𝟙 X) a₁ (𝟙 Z) (by simp) <| Category.comp_id _)
      (pullback.map _ _ _ _ (𝟙 X) a₂ (𝟙 Z) (by simp) <| (Category.comp_id _).trans h.1.1)

Proof state:
case refine_5
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ (fun s => pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
      s =
    m

Tactic:
apply pullback.hom_ext

Result:
case refine_5.h₀
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ (fun s => pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
        s ≫
      pullback.fst f (a₁ ≫ g) =
    m ≫ pullback.fst f (a₁ ≫ g)

case refine_5.h₁
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ (fun s => pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
        s ≫
      pullback.snd f (a₁ ≫ g) =
    m ≫ pullback.snd f (a₁ ≫ g)

Proof state:
case refine_5.h₁
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ (fun s => pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
        s ≫
      pullback.snd f (a₁ ≫ g) =
    m ≫ pullback.snd f (a₁ ≫ g)

Tactic:
symm

Result:
case refine_5.h₁
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ m ≫ pullback.snd f (a₁ ≫ g) =
    (fun s =>
          pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
        s ≫
      pullback.snd f (a₁ ≫ g)

Proof state:
case refine_5.h₁.h₀
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ (m ≫ pullback.snd f (a₁ ≫ g)) ≫ (IsPullback.cone h).fst =
    ((fun s =>
            pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
          s ≫
        pullback.snd f (a₁ ≫ g)) ≫
      (IsPullback.cone h).fst

Tactic:
simpa using hm WalkingCospan.left =≫ pullback.snd f g

Result:
no goals

Proof state:
case refine_5.h₁.h₁
C : Type u
inst✝² : Category.{v, u} C
X Y Z A : C
g : Y ⟶ Z
a₁ a₂ : A ⟶ Y
h : IsKernelPair g a₁ a₂
f : X ⟶ Z
inst✝¹ : HasPullback f g
inst✝ : HasPullback f (a₁ ≫ g)
s : PullbackCone (pullback.fst f g) (pullback.fst f g)
m :
  s.pt ⟶
    (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯) (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯)
        ⋯).pt
hm :
  ∀ (j : WalkingCospan),
    m ≫
        (PullbackCone.mk (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₁ (𝟙 Z) ⋯ ⋯)
                (pullback.map f (a₁ ≫ g) f g (𝟙 X) a₂ (𝟙 Z) ⋯ ⋯) ⋯).π.app
          j =
      s.π.app j
⊢ (m ≫ pullback.snd f (a₁ ≫ g)) ≫ (IsPullback.cone h).snd =
    ((fun s =>
            pullback.lift (s.fst ≫ pullback.fst f g) (h.lift (s.fst ≫ pullback.snd f g) (s.snd ≫ pullback.snd f g) ⋯) ⋯)
          s ≫
        pullback.snd f (a₁ ≫ g)) ≫
      (IsPullback.cone h).snd

Tactic:
simpa using hm WalkingCospan.right =≫ pullback.snd f g

Result:
no goals