-- Lean 4 proof step from Mathlib.CategoryTheory.Sites.Preserves (goal)
Goal:
C : Type u
inst : Category.{v, u} C
I : C
F : Functor.{v, w, u, w + 1} Cᵒᵖ (Type w)
hF :
  IsSheafFor (X := I) F
    (ofArrows Empty.elim fun (a : Empty) ↦ Empty.instIsEmpty.elim (p := fun (x : Empty) ↦ x.elim ⟶ I) a)
hI : IsInitial.{v, u} I
α : Type u_1
inst_1 : Small.{w, u_1} α
X : α → C
c : Cofan.{u_1, v, u} X
inst_2 : (ofArrows X c.inj).HasPairwisePullbacks
inst_3 : HasInitial.{v, u} C
inst_4 : ∀ (i : α), Mono (c.inj i)
hd :
  Pairwise fun (i j : α) ↦ IsPullback (P := initial.{v, u} C) (initial.to (X i)) (initial.to (X j)) (c.inj i) (c.inj j)
i j : α
a : Equalizer.Presieve.Arrows.FirstObj F X
hi : i = j
pos :
  Eq (α := F.obj (op (pullback (c.inj j) (c.inj j))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (Quiver.Hom.op (pullback.fst (c.inj j) (c.inj j)))) :
          (a : F.obj (op (X j))) → F.obj (op (pullback (c.inj j) (c.inj j))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (Pi.π (fun (i : α) ↦ F.obj (op (X i))) j) :
              (a : Equalizer.Presieve.Arrows.FirstObj F X) → F.obj (op (X j)))
            a :
          F.obj (op (X j))) :
      F.obj (op (pullback (c.inj j) (c.inj j))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (Quiver.Hom.op (pullback.snd (c.inj j) (c.inj j)))) :
          (a : F.obj (op (X j))) → F.obj (op (pullback (c.inj j) (c.inj j))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (Pi.π (fun (i : α) ↦ F.obj (op (X i))) j) :
              (a : Equalizer.Presieve.Arrows.FirstObj F X) → F.obj (op (X j)))
            a :
          F.obj (op (X j))) :
      F.obj (op (pullback (c.inj j) (c.inj j))))
⊢ Eq (α := F.obj (op (pullback (c.inj i) (c.inj j))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (Quiver.Hom.op (pullback.fst (c.inj i) (c.inj j)))) :
          (a : F.obj (op (X i))) → F.obj (op (pullback (c.inj i) (c.inj j))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (Pi.π (fun (i : α) ↦ F.obj (op (X i))) i) :
              (a : Equalizer.Presieve.Arrows.FirstObj F X) → F.obj (op (X i)))
            a :
          F.obj (op (X i))) :
      F.obj (op (pullback (c.inj i) (c.inj j))))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (Quiver.Hom.op (pullback.snd (c.inj i) (c.inj j)))) :
          (a : F.obj (op (X j))) → F.obj (op (pullback (c.inj i) (c.inj j))))
        ((ConcreteCategory.hom (FC := TypeCat.Fun) (Pi.π (fun (i : α) ↦ F.obj (op (X i))) j) :
              (a : Equalizer.Presieve.Arrows.FirstObj F X) → F.obj (op (X j)))
            a :
          F.obj (op (X j))) :
      F.obj (op (pullback (c.inj i) (c.inj j))))
have pos₁ : Eq (α := F.obj (op (pullback (c.inj j) (c.inj j))))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (Quiver.Hom.op (pullback.snd (c.inj j) (c.inj j)))) :
        (a : F.obj (op (X j))) → F.obj (op (pullback (c.inj j) (c.inj j))))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (Pi.π (fun (i : α) ↦ F.obj (op (X i))) j) :
            (a : Equalizer.Presieve.Arrows.FirstObj F X) → F.obj (op (X j)))
          a :
        F.obj (op (X j))) :
    F.obj (op (pullback (c.inj j) (c.inj j))))
  ((ConcreteCategory.hom (FC := TypeCat.Fun) (F.map (Quiver.Hom.op (pullback.snd (c.inj j) (c.inj j)))) :
        (a : F.obj (op (X j))) → F.obj (op (pullback (c.inj j) (c.inj j))))
      ((ConcreteCategory.hom (FC := TypeCat.Fun) (Pi.π (fun (i : α) ↦ F.obj (op (X i))) j) :
            (a : Equalizer.Presieve.Arrows.FirstObj F X) → F.obj (op (X j)))
          a :
        F.obj (op (X j))) :
    F.obj (op (pullback (c.inj j) (c.inj j))))
Tactic: rw [hi, Mono.right_cancellation _ _ pullback.condition]