/-- The pullback of two monomorphisms exists. -/
@[irreducible, nolint defLemma] -- Porting note: changed to irreducible and a def
def pullback_of_mono {X Y Z : C} (a : X ⟶ Z) (b : Y ⟶ Z) [Mono a] [Mono b] :
    HasLimit (cospan a b) :=
  let ⟨P, f, haf, i⟩ := normalMonoOfMono a
  let ⟨Q, g, hbg, i'⟩ := normalMonoOfMono b
  let ⟨a', ha'⟩ :=
    KernelFork.IsLimit.lift' i (kernel.ι (prod.lift f g)) <|
      calc
        kernel.ι (prod.lift f g) ≫ f = kernel.ι (prod.lift f g) ≫ prod.lift f g ≫ Limits.prod.fst :=
          by rw [prod.lift_fst]
        _ = (0 : kernel (prod.lift f g) ⟶ P ⨯ Q) ≫ Limits.prod.fst := by rw [kernel.condition_assoc]
        _ = 0 := zero_comp

  let ⟨b', hb'⟩ :=
    KernelFork.IsLimit.lift' i' (kernel.ι (prod.lift f g)) <|
      calc
        kernel.ι (prod.lift f g) ≫ g = kernel.ι (prod.lift f g) ≫ prod.lift f g ≫ Limits.prod.snd :=
          by rw [prod.lift_snd]
        _ = (0 : kernel (prod.lift f g) ⟶ P ⨯ Q) ≫ Limits.prod.snd := by rw [kernel.condition_assoc]
        _ = 0 := zero_comp

  HasLimit.mk
    { cone :=
        PullbackCone.mk a' b' <| by
          simp? at ha' hb' says
            simp only [parallelPair_obj_zero, Fork.ofι_pt, Fork.ι_ofι] at ha' hb'
          rw [ha', hb']
      isLimit :=
        PullbackCone.IsLimit.mk _
          (fun s =>
            kernel.lift (prod.lift f g) (PullbackCone.snd s ≫ b) <|
              prod.hom_ext
                (calc
                  ((PullbackCone.snd s ≫ b) ≫ prod.lift f g) ≫ Limits.prod.fst =
                      PullbackCone.snd s ≫ b ≫ f :=
                    by simp only [prod.lift_fst, Category.assoc]
                  _ = PullbackCone.fst s ≫ a ≫ f := by rw [PullbackCone.condition_assoc]
                  _ = PullbackCone.fst s ≫ 0 := by rw [haf]
                  _ = 0 ≫ Limits.prod.fst := by rw [comp_zero, zero_comp]
                  )
                (calc
                  ((PullbackCone.snd s ≫ b) ≫ prod.lift f g) ≫ Limits.prod.snd =
                      PullbackCone.snd s ≫ b ≫ g :=
                    by simp only [prod.lift_snd, Category.assoc]
                  _ = PullbackCone.snd s ≫ 0 := by rw [hbg]
                  _ = 0 ≫ Limits.prod.snd := by rw [comp_zero, zero_comp]
                  ))
          (fun s =>
            (cancel_mono a).1 <| by
              rw [KernelFork.ι_ofι] at ha'
              simp [ha', PullbackCone.condition s])
          (fun s =>
            (cancel_mono b).1 <| by
              rw [KernelFork.ι_ofι] at hb'
              

Proof state:
C : Type u_1
inst✝⁶ : Category.{?u.100, u_1} C
inst✝⁵ : HasZeroMorphisms C
inst✝⁴ : HasFiniteProducts C
inst✝³ : HasKernels C
inst✝² : NormalMonoCategory C
X Y Z : C
a : X ⟶ Z
b : Y ⟶ Z
inst✝¹ : Mono a
inst✝ : Mono b
P : C
f : Z ⟶ P
haf : a ≫ f = 0
i : IsLimit (KernelFork.ofι a haf)
Q : C
g : Z ⟶ Q
hbg : b ≫ g = 0
i' : IsLimit (KernelFork.ofι b hbg)
a' : kernel (prod.lift f g) ⟶ (KernelFork.ofι a haf).pt
ha' : a' ≫ Fork.ι (KernelFork.ofι a haf) = kernel.ι (prod.lift f g)
b' : kernel (prod.lift f g) ⟶ (KernelFork.ofι b hbg).pt
hb' : b' ≫ b = kernel.ι (prod.lift f g)
s : PullbackCone a b
⊢ ((fun s => kernel.lift (prod.lift f g) (PullbackCone.snd s ≫ b) (_ : (PullbackCone.snd s ≫ b) ≫ prod.lift f g = 0))
          s ≫
        b') ≫
      b =
    PullbackCone.snd s ≫ b

Next tactic:
simp [hb']