-- Lean 4 proof step from Mathlib.CategoryTheory.SmallObject.Iteration.ExtendToSucc (goal)
Goal:
C : Type u_1
inst : Category.{v_1, u_1} C
J : Type u
inst_1 : LinearOrder J
inst_2 : SuccOrder J
j : J
hj : ¬IsMax j
F : Functor.{u, v_1, u, u_1} (↑(Set.Iic j) : Type u) C
X : C
τ : F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯) ⟶ X
rw :
  Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) j ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯))
    (if h₂ : j ≤ j then
      CategoryStruct.comp (Y := F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j h₂))
        (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j h₂)))
        (F.map (𝟙 (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j h₂)) ≫
          τ ≫
            Iso.inv (objSuccIso hj F X) ≫
              𝟙 (obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯)))
    else eqToHom ⋯)
    (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫ τ ≫ Iso.inv (objSuccIso hj F X))
rw₁ :
  Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) j ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯))
    (CategoryStruct.comp (Y := F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯))
      (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)))
      (F.map (𝟙 (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫
        τ ≫
          Iso.inv (objSuccIso hj F X) ≫
            𝟙 (obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯))))
    (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫ τ ≫ Iso.inv (objSuccIso hj F X))
rw₂ :
  Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) j ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯))
    (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫
      𝟙 (F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫
        τ ≫
          Iso.inv (objSuccIso hj F X) ≫
            𝟙 (obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯)))
    (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫ τ ≫ Iso.inv (objSuccIso hj F X))
⊢ Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) j ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯))
    (if h₁ : Order.succ j ≤ j then
      CategoryStruct.comp (Y := F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯))
        (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)))
        (CategoryStruct.comp (Y := F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) (Order.succ j) h₁))
          (F.map (homOfLE ⋯)) (Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) (Order.succ j) h₁))))
    else
      if h₂ : j ≤ j then
        CategoryStruct.comp (Y := F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j h₂))
          (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j h₂)))
          (F.map (𝟙 (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j h₂)) ≫
            τ ≫
              Iso.inv (objSuccIso hj F X) ≫
                𝟙 (obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) ⋯)))
      else eqToHom ⋯)
    (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j ⋯)) ≫ τ ≫ Iso.inv (objSuccIso hj F X))
have rw₃ : Eq (α :=
  obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) j sorry) ⟶
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) (Order.succ j) sorry))
  (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j sorry)) ≫
    𝟙 (F.obj (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j sorry)) ≫ τ ≫ Iso.inv (objSuccIso hj F X))
  (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) j sorry)) ≫ τ ≫ Iso.inv (objSuccIso hj F X))
Tactic: rw [dif_neg (by simpa only [Order.succ_le_iff_isMax] using hj),
    dif_pos (by rfl), Functor.map_id, comp_id, id_comp]