-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.DayConvolution (hypothesis)
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
F G : Functor.{v₁, v₂, u₁, u₂} C V
inst_4 : DayConvolution F G
inst_5 :
  ∀ (v : V) (d : C), Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (tensor C) d) (tensorLeft v)
inst_6 :
  ∀ (v : V) (d : C), Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (tensor C) d) (tensorRight v)
inst_7 :
  ∀ (v : V) (d : C × C),
    Limits.PreservesColimitsOfShape (CostructuredArrow.{v₁, v₁, u₁, u₁} (Functor.prod (tensor C) (𝟭 C)) d)
      (tensorRight v)
H K : Functor.{v₁, v₂, u₁, u₂} C V
inst_8 : DayConvolution G H
inst_9 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 F G _) H
inst_10 : DayConvolution F (@convolution _ _ _ _ inst_2 inst_3 G H _)
inst_11 : DayConvolution H K
inst_12 : DayConvolution G (@convolution _ _ _ _ inst_2 inst_3 H K _)
inst_13 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 G H _) K
inst_14 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _) K
inst_15 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 F G _) (@convolution _ _ _ _ inst_2 inst_3 H K _)
inst_16 : DayConvolution (@convolution _ _ _ _ inst_2 inst_3 F (@convolution _ _ _ _ inst_2 inst_3 G H _) _) K
inst_17 : DayConvolution F (@convolution _ _ _ _ inst_2 inst_3 G (@convolution _ _ _ _ inst_2 inst_3 H K _) _)
inst_18 : DayConvolution F (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 G H _) K _)
⊢ Eq (α :=
    (@convolution _ _ _ _ inst_2 inst_3 F G _ ⊠ H) ⊠ K ⟶
      Functor.prod (tensor C) (𝟭 C) ⋙
        tensor C ⋙
          @convolution _ _ _ _ inst_2 inst_3 F
            (@convolution _ _ _ _ inst_2 inst_3 G (@convolution _ _ _ _ inst_2 inst_3 H K _) _) _)
    (extensionUnitLeft (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _)
        (unit (@convolution _ _ _ _ inst_2 inst_3 F G _) H) K ≫
      Functor.whiskerLeft (Functor.prod (tensor C) (𝟭 C))
        (CategoryStruct.comp (Y :=
          tensor C ⋙
            @convolution _ _ _ _ inst_2 inst_3
              (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _) K _)
          (unit (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _) K)
          (Functor.whiskerLeft (tensor C)
            (CategoryStruct.comp (Y :=
              @convolution _ _ _ _ inst_2 inst_3
                (@convolution _ _ _ _ inst_2 inst_3 F (@convolution _ _ _ _ inst_2 inst_3 G H _) _) K _)
              (map (Iso.hom (associator F G H)) (𝟙 K))
              (CategoryStruct.comp (Y :=
                @convolution _ _ _ _ inst_2 inst_3 F
                  (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 G H _) K _) _)
                (Iso.hom (associator F (@convolution _ _ _ _ inst_2 inst_3 G H _) K))
                (map (𝟙 F) (Iso.hom (associator G H K))))))))
    (extensionUnitLeft (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _)
        (unit (@convolution _ _ _ _ inst_2 inst_3 F G _) H) K ≫
      Functor.whiskerLeft (Functor.prod (tensor C) (𝟭 C))
        (CategoryStruct.comp (Y :=
          tensor C ⋙
            @convolution _ _ _ _ inst_2 inst_3
              (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _) K _)
          (unit (@convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _) H _) K)
          (Functor.whiskerLeft (tensor C)
            (CategoryStruct.comp (Y :=
              @convolution _ _ _ _ inst_2 inst_3 (@convolution _ _ _ _ inst_2 inst_3 F G _)
                (@convolution _ _ _ _ inst_2 inst_3 H K _) _)
              (Iso.hom (associator (@convolution _ _ _ _ inst_2 inst_3 F G _) H K))
              (Iso.hom (associator F G (@convolution _ _ _ _ inst_2 inst_3 H K _)))))))
have this : ((@convolution _ _ _ _ inst_2 inst_3 F G _ ⊠ H) ⊠ K).IsLeftKanExtension
  (extensionUnitLeft (@convolution _ _ _ _ inst_2 inst_3 F G _ ⊠ H)
    (extensionUnitLeft (@convolution _ _ _ _ inst_2 inst_3 F G _) (unit F G) H) K)
Tactic: have : (((F ⊛ G) ⊠ H) ⊠ K).IsLeftKanExtension
    (α := extensionUnitLeft ((F ⊛ G) ⊠ H)
      (extensionUnitLeft _ (unit F G) H) K) :=
    isPointwiseLeftKanExtensionExtensionUnitLeft _ _ _
      (isPointwiseLeftKanExtensionExtensionUnitLeft _ _ _
        (isPointwiseLeftKanExtensionUnit F G)) |>.isLeftKanExtension