-- 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
i₁ i₂ i₃ : J
h₁₂ : i₁ ≤ i₂
h₂₃ : i₂ ≤ i₃
h : i₃ ≤ Order.succ j
h₁ : i₃ ≤ j
pos :
  Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
    (map hj F τ i₁ i₃ ⋯ h :
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
        obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
    ((Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₁ ⋯)) ≫
          F.map (homOfLE h₁₂) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ ⋯))) ≫
        map hj F τ i₂ i₃ h₂₃ h :
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
        obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
pos₁ :
  Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
    (map hj F τ i₁ i₃ ⋯ h :
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
        obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
    (CategoryStruct.comp (Y := obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₂ ⋯))
        (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₁ ⋯)) ≫
          F.map (homOfLE h₁₂) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ ⋯)))
        (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ ⋯)) ≫
          F.map (homOfLE h₂₃) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₃ h₁))) :
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
        obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
pos₂ :
  Eq (α :=
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ ⋯) ⟶
      obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
    (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₁ ⋯)) ≫
      F.map (homOfLE ⋯) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₃ h₁)))
    (CategoryStruct.comp (Y := obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₂ ⋯))
      (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₁ ⋯)) ≫
        F.map (homOfLE h₁₂) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ ⋯)))
      (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ ⋯)) ≫
        F.map (homOfLE h₂₃) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₃ h₁))))
⊢ map hj F τ i₁ i₃ ⋯ h = map hj F τ i₁ i₂ h₁₂ ⋯ ≫ map hj F τ i₂ i₃ h₂₃ h
have pos₃ : Eq (α :=
  obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₁ sorry) ⟶
    obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₃ h))
  (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₁ sorry)) ≫
    F.map (homOfLE sorry) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₃ h₁)))
  (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₁ sorry)) ≫
    CategoryStruct.comp (Y := obj F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic (Order.succ j)) i₂ sorry))
      (F.map (homOfLE h₁₂) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ sorry)))
      (Iso.hom (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₂ sorry)) ≫
        F.map (homOfLE h₂₃) ≫ Iso.inv (objIso F X (Subtype.mk (p := fun (x : J) ↦ x ∈ Set.Iic j) i₃ h₁))))
Tactic: rw [map_eq hj F τ i₁ i₂ _ (h₂₃.trans h₁), map_eq hj F τ i₂ i₃ _ h₁,
      map_eq hj F τ i₁ i₃ _ h₁, assoc, assoc, Iso.inv_hom_id_assoc, ← Functor.map_comp_assoc,
      homOfLE_comp]