-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.DayConvolution (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
V : Type u₂
inst_1 : Category.{v₂, u₂} V
inst_2 : MonoidalCategory.{v₁, u₁} C
inst_3 : MonoidalCategory.{v₂, u₂} V
D : Type u₃
inst_4 : Category.{v₃, u₃} D
inst_5 : MonoidalCategoryStruct.{v₃, u₃} D
inst_6 : LawfulDayConvolutionMonoidalCategoryStruct.{v₁, v₂, v₃, u₁, u₂, u₃} C V D
d : D
inst_7 :
  ∀ (v : V) (d : C),
    Limits.PreservesColimitsOfShape.{0, v₁, v₂, v₂, u₂, u₂}
      (CostructuredArrow (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst))) d) (tensorRight v)
⊢ Eq (α :=
    Quiver.Hom (V := Functor.{v₁, v₂, u₁, u₂} C V)
      (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _)
        (tensorObj.{v₃, u₃} (𝒞 := inst_4) (tensorUnit.{v₃, u₃} D (𝒞 := inst_4)) d))
      (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d))
    (Functor.map (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) (Iso.hom (λ_ d)))
    (Iso.hom
      (DayConvolutionUnit.leftUnitor
        (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) (tensorUnit.{v₃, u₃} D (𝒞 := inst_4)))
        (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d)))
have a : Eq (α :=
  Functor.obj (C := Functor.{v₁, v₂, u₁, u₂} C V)
    (Functor.obj (Functor.whiskeringLeft (C × C) C V) (tensor C) ⋙
      Functor.obj (Functor.whiskeringLeft (Discrete PUnit.{1} × C) (C × C) V)
          (Functor.prod (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst))) (𝟭 C)) ⋙
        Functor.obj coyoneda
          (Opposite.op
            (Functor.fromPUnit (tensorUnit.{v₂, u₂} V (𝒞 := inst_1)) ⊠
              Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d)))
    (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d))
  ((Functor.CorepresentableBy.homEquiv
          (corepresentableByLeft
            (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) (tensorUnit.{v₃, u₃} D (𝒞 := inst_4)))
            (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d)) :
        (a : (ι C V D).obj (𝟙_ D) ⊛ (ι C V D).obj d ⟶ (ι C V D).obj d) →
          ((Functor.whiskeringLeft (C × C) C V).obj (tensor C) ⋙
                (Functor.whiskeringLeft (Discrete PUnit.{1} × C) (C × C) V).obj
                    ((Functor.fromPUnit (𝟙_ C)).prod (𝟭 C)) ⋙
                  coyoneda.obj (Opposite.op (Functor.fromPUnit (𝟙_ V) ⊠ (ι C V D).obj d))).obj
            ((ι C V D).obj d))
      (Functor.map (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) (Iso.hom (λ_ d))) :
    Functor.obj (C := Functor.{v₁, v₂, u₁, u₂} C V)
      (Functor.obj (Functor.whiskeringLeft (C × C) C V) (tensor C) ⋙
        Functor.obj (Functor.whiskeringLeft (Discrete PUnit.{1} × C) (C × C) V)
            (Functor.prod (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst))) (𝟭 C)) ⋙
          Functor.obj coyoneda
            (Opposite.op
              (Functor.fromPUnit (tensorUnit.{v₂, u₂} V (𝒞 := inst_1)) ⊠
                Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d)))
      (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d))
  ((Functor.CorepresentableBy.homEquiv
          (corepresentableByLeft
            (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) (tensorUnit.{v₃, u₃} D (𝒞 := inst_4)))
            (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d)) :
        (a : (ι C V D).obj (𝟙_ D) ⊛ (ι C V D).obj d ⟶ (ι C V D).obj d) →
          ((Functor.whiskeringLeft (C × C) C V).obj (tensor C) ⋙
                (Functor.whiskeringLeft (Discrete PUnit.{1} × C) (C × C) V).obj
                    ((Functor.fromPUnit (𝟙_ C)).prod (𝟭 C)) ⋙
                  coyoneda.obj (Opposite.op (Functor.fromPUnit (𝟙_ V) ⊠ (ι C V D).obj d))).obj
            ((ι C V D).obj d))
      (Iso.hom
        (DayConvolutionUnit.leftUnitor
          (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) (tensorUnit.{v₃, u₃} D (𝒞 := inst_4)))
          (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d))) :
    Functor.obj (C := Functor.{v₁, v₂, u₁, u₂} C V)
      (Functor.obj (Functor.whiskeringLeft (C × C) C V) (tensor C) ⋙
        Functor.obj (Functor.whiskeringLeft (Discrete PUnit.{1} × C) (C × C) V)
            (Functor.prod (Functor.fromPUnit (tensorUnit.{v₁, u₁} C (𝒞 := inst))) (𝟭 C)) ⋙
          Functor.obj coyoneda
            (Opposite.op
              (Functor.fromPUnit (tensorUnit.{v₂, u₂} V (𝒞 := inst_1)) ⊠
                Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d)))
      (Functor.obj (@ι C _ V _ inst_2 inst_3 D _ inst_5 _) d))
Tactic: apply corepresentableByLeft
    (ι C V D |>.obj <| 𝟙_ D) (ι C V D |>.obj d) |>.homEquiv.injective