-- Module: Mathlib.CategoryTheory.Shift.Pullback
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
A : Type u_2
B : Type u_3
inst_1 : AddMonoid A
inst_2 : AddMonoid B
inst_3 : HasShift.{v_1, u_1, u_3} C B
φ : A →+ B
X : PullbackShift.{v_1, u_1, u_2, u_3} C φ
a₁ a₂ a₃ : A
h : a₁ + a₂ = a₃
b₁ b₂ b₃ : B
h₁ : b₁ = (φ : (a : A) → B) a₁
h₂ : b₂ = (φ : (a : A) → B) a₂
h₃ : b₃ = (φ : (a : A) → B) a₃
D : Type u_4
inst_4 : Category.{v_2, u_4} D
inst_5 : HasShift.{v_2, u_4, u_3} D B
F : Functor.{v_1, v_2, u_1, u_4} C D
inst_6 : F.CommShift B
x : PullbackShift.{v_1, u_1, u_2, u_3} C φ
⊢ Eq (α :=
    Quiver.Hom (V := PullbackShift.{v_2, u_4, u_2, u_3} D φ)
      (obj (PullbackShift.functor φ F)
        (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_2} (PullbackShift.{v_1, u_1, u_2, u_3} C φ) (A := A) 0) x))
      (obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_2} (PullbackShift.{v_2, u_4, u_2, u_3} D φ) (A := A) 0)
        (obj (PullbackShift.functor φ F) x)))
    (CategoryStruct.comp (Y :=
      F.obj (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_3} C (A := B) ((φ : (a : A) → B) 0 : B)) x))
      (F.map (NatTrans.app (Iso.hom (pullbackShiftIso C φ 0 ((φ : (a : A) → B) 0) ⋯)) x) :
        F.obj
            (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_2} (PullbackShift.{v_1, u_1, u_2, u_3} C φ) (A := A) 0)
              x) ⟶
          F.obj (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_3} C (A := B) ((φ : (a : A) → B) 0 : B)) x))
      (F.map
            (NatTrans.app (Iso.hom (shiftFunctorZero' (PullbackShift.{v_1, u_1, u_2, u_3} C φ) ((φ : (a : A) → B) 0) ⋯))
              x) ≫
          CategoryStruct.comp (Y :=
            obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_3} D (A := B) ((φ : (a : A) → B) 0 : B)) (F.obj x))
            (NatTrans.app (Iso.inv (shiftFunctorZero' (PullbackShift.{v_2, u_4, u_2, u_3} D φ) ((φ : (a : A) → B) 0) ⋯))
                (F.obj x) :
              obj.{v_2, v_2, u_4, u_4} (𝟭 (PullbackShift.{v_2, u_4, u_2, u_3} D φ)) (F.obj x) ⟶
                obj.{v_2, v_2, u_4, u_4}
                  (shiftFunctor.{v_2, u_4, u_3} (PullbackShift.{v_2, u_4, u_2, u_3} D φ) (A := B)
                    ((φ : (a : A) → B) 0 : B))
                  (F.obj x))
            (NatTrans.app (Iso.inv (pullbackShiftIso D φ 0 ((φ : (a : A) → B) 0) ⋯)) (F.obj x) :
              obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_3} D (A := B) ((φ : (a : A) → B) 0 : B)) (F.obj x) ⟶
                obj.{v_2, v_2, u_4, u_4}
                  (shiftFunctor.{v_2, u_4, u_2} (PullbackShift.{v_2, u_4, u_2, u_3} D φ) (A := A) 0) (F.obj x)) :
        F.obj (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_3} C (A := B) ((φ : (a : A) → B) 0 : B)) x) ⟶
          obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_2} (PullbackShift.{v_2, u_4, u_2, u_3} D φ) (A := A) 0)
            (obj (PullbackShift.functor φ F) x)))
    (CategoryStruct.comp (Y :=
      obj (PullbackShift.functor φ F)
        (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_3} C (A := B) ((φ : (a : A) → B) 0 : B)) x))
      (map (PullbackShift.functor φ F) (NatTrans.app (Iso.hom (pullbackShiftIso C φ 0 ((φ : (a : A) → B) 0) ⋯)) x) :
        Quiver.Hom (V := PullbackShift.{v_2, u_4, u_2, u_3} D φ)
          (obj (PullbackShift.functor φ F)
            (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_2} (PullbackShift.{v_1, u_1, u_2, u_3} C φ) (A := A) 0)
              x))
          (obj (PullbackShift.functor φ F)
            (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_3} C (A := B) ((φ : (a : A) → B) 0 : B)) x)))
      (map (PullbackShift.functor φ F) (NatTrans.app (Iso.hom (shiftFunctorZero' C ((φ : (a : A) → B) 0) ⋯)) x) ≫
          CategoryStruct.comp (Y :=
            obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_3} D (A := B) ((φ : (a : A) → B) 0 : B))
              (obj (PullbackShift.functor φ F) x))
            (NatTrans.app (Iso.inv (shiftFunctorZero' D ((φ : (a : A) → B) 0) ⋯)) (obj (PullbackShift.functor φ F) x) :
              obj.{v_2, v_2, u_4, u_4} (𝟭 D) (obj (PullbackShift.functor φ F) x) ⟶
                obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_3} D (A := B) ((φ : (a : A) → B) 0 : B))
                  (obj (PullbackShift.functor φ F) x))
            (NatTrans.app (Iso.inv (pullbackShiftIso D φ 0 ((φ : (a : A) → B) 0) ⋯))
                (obj (PullbackShift.functor φ F) x) :
              obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_3} D (A := B) ((φ : (a : A) → B) 0 : B))
                  (obj (PullbackShift.functor φ F) x) ⟶
                obj.{v_2, v_2, u_4, u_4}
                  (shiftFunctor.{v_2, u_4, u_2} (PullbackShift.{v_2, u_4, u_2, u_3} D φ) (A := A) 0)
                  (obj (PullbackShift.functor φ F) x)) :
        Quiver.Hom (V := PullbackShift.{v_2, u_4, u_2, u_3} D φ)
          (obj (PullbackShift.functor φ F)
            (obj.{v_1, v_1, u_1, u_1} (shiftFunctor.{v_1, u_1, u_3} C (A := B) ((φ : (a : A) → B) 0 : B)) x))
          (obj.{v_2, v_2, u_4, u_4} (shiftFunctor.{v_2, u_4, u_2} (PullbackShift.{v_2, u_4, u_2, u_3} D φ) (A := A) 0)
            (obj (PullbackShift.functor φ F) x))))

Tactic:
rfl