-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Functor (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
inst_1 : MonoidalCategory.{v₁, u₁} C
D : Type u₂
inst_2 : Category.{v₂, u₂} D
inst_3 : MonoidalCategory.{v₂, u₂} D
F : Functor.{v₁, v₂, u₁, u₂} C D
inst_4 : F.OplaxMonoidal
X : C
rw :
  Eq (α := F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶ F.obj X)
    (CategoryStruct.comp (Y := tensorObj.{v₂, u₂} (𝒞 := inst_2) (tensorUnit.{v₂, u₂} D (𝒞 := inst_2)) (F.obj X))
      (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X) (Iso.hom (λ_ (F.obj X))))
    (F.map (Iso.hom (λ_ X)))
rw₁ :
  Eq (α :=
    F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶
      tensorObj.{v₂, u₂} (𝒞 := inst_2) (tensorUnit.{v₂, u₂} D (𝒞 := inst_2)) (F.obj X))
    (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X) (F.map (Iso.hom (λ_ X)) ≫ Iso.inv (λ_ (F.obj X)))
rw₂ :
  Eq (α :=
    F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶
      tensorObj.{v₂, u₂} (𝒞 := inst_2) (tensorUnit.{v₂, u₂} D (𝒞 := inst_2)) (F.obj X))
    (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X)
    (F.map (Iso.hom (λ_ X)) ≫
      CategoryStruct.comp (Y := F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X))
        (F.map (Iso.inv (λ_ X))) (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X))
rw₃ :
  Eq (α :=
    F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶
      tensorObj.{v₂, u₂} (𝒞 := inst_2) (tensorUnit.{v₂, u₂} D (𝒞 := inst_2)) (F.obj X))
    (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X)
    (CategoryStruct.comp (Y := F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X))
      (F.map (Iso.hom (λ_ X)) ≫ F.map (Iso.inv (λ_ X))) (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X))
rw₄ :
  Eq (α :=
    F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶
      tensorObj.{v₂, u₂} (𝒞 := inst_2) (tensorUnit.{v₂, u₂} D (𝒞 := inst_2)) (F.obj X))
    (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X)
    (CategoryStruct.comp (Y := F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X))
      (F.map (Iso.hom (λ_ X) ≫ Iso.inv (λ_ X))) (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X))
⊢ Eq (α := F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶ F.obj X)
    (CategoryStruct.comp (Y := tensorObj.{v₂, u₂} (𝒞 := inst_2) (F.obj (tensorUnit.{v₁, u₁} C (𝒞 := inst))) (F.obj X))
      (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) (η F ▷ F.obj X ≫ Iso.hom (λ_ (F.obj X))))
    (F.map (Iso.hom (λ_ X)))
have rw₅ : Eq (α :=
  F.obj (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X) ⟶
    tensorObj.{v₂, u₂} (𝒞 := inst_2) (tensorUnit.{v₂, u₂} D (𝒞 := inst_2)) (F.obj X))
  (δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X)
  (F.map (𝟙 (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X)) ≫
    δ F (tensorUnit.{v₁, u₁} C (𝒞 := inst)) X ≫ η F ▷ F.obj X)
Tactic: rw [← Category.assoc, ← Iso.eq_comp_inv, left_unitality, ← Category.assoc,
    ← F.map_comp, Iso.hom_inv_id, F.map_id, id_comp]