-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Rigid.Basic (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
inst_1 : MonoidalCategory.{v₁, u₁} C
X Y Z : C
inst_2 : HasRightDual.{v₁, u₁} X
inst_3 : HasRightDual.{v₁, u₁} Y
inst_4 : HasRightDual.{v₁, u₁} Z
f : X ⟶ Y
g : Y ⟶ Z
⊢ Eq (α := tensorUnit.{v₁, u₁} C (𝒞 := inst) ⟶ tensorObj.{v₁, u₁} (𝒞 := inst) Z (@rightDual.{v₁, u₁} _ inst inst_1 X _))
    (η_ Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ≫
      (g ⊗ₘ
        CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (@rightDual.{v₁, u₁} _ inst inst_1 Y _) (tensorUnit.{v₁, u₁} C (𝒞 := inst)))
          (Iso.inv (ρ_ (@rightDual.{v₁, u₁} _ inst inst_1 Y _)))
          (@rightDual.{v₁, u₁} _ inst inst_1 Y _ ◁ η_ X (@rightDual.{v₁, u₁} _ inst inst_1 X _) ≫
            @rightDual.{v₁, u₁} _ inst inst_1 Y _ ◁ f ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _ ≫
              Iso.inv (α_ (@rightDual.{v₁, u₁} _ inst inst_1 Y _) Y (@rightDual.{v₁, u₁} _ inst inst_1 X _)) ≫
                ε_ Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _ ≫
                  Iso.hom (λ_ (@rightDual.{v₁, u₁} _ inst inst_1 X _)))))
    (η_ X (@rightDual.{v₁, u₁} _ inst inst_1 X _) ≫
      f ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _ ≫ g ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _)
have _ : η_ Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ≫
    (g ⊗ₘ
      CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (@rightDual.{v₁, u₁} _ inst inst_1 Y _) (tensorUnit.{v₁, u₁} C (𝒞 := inst)))
        (Iso.inv (ρ_ (@rightDual.{v₁, u₁} _ inst inst_1 Y _)))
        (@rightDual.{v₁, u₁} _ inst inst_1 Y _ ◁ η_ X (@rightDual.{v₁, u₁} _ inst inst_1 X _) ≫
          @rightDual.{v₁, u₁} _ inst inst_1 Y _ ◁ f ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _ ≫
            Iso.inv (α_ (@rightDual.{v₁, u₁} _ inst inst_1 Y _) Y (@rightDual.{v₁, u₁} _ inst inst_1 X _)) ≫
              ε_ Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _ ≫
                Iso.hom (λ_ (@rightDual.{v₁, u₁} _ inst inst_1 X _)))) =
  monoidalComp (Y :=
    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorUnit.{v₁, u₁} C (𝒞 := inst)) (tensorUnit.{v₁, u₁} C (𝒞 := inst)))
    (𝟙 (tensorUnit.{v₁, u₁} C (𝒞 := inst)))
    (monoidalComp (X :=
      tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _))
        (tensorObj.{v₁, u₁} (𝒞 := inst) X (@rightDual.{v₁, u₁} _ inst inst_1 X _)))
      (Y :=
      tensorObj.{v₁, u₁} (𝒞 := inst) Y
        (tensorObj.{v₁, u₁} (𝒞 := inst) (@rightDual.{v₁, u₁} _ inst inst_1 Y _)
          (tensorObj.{v₁, u₁} (𝒞 := inst) X (@rightDual.{v₁, u₁} _ inst inst_1 X _))))
      (η_ Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ▷ tensorUnit.{v₁, u₁} C (𝒞 := inst) ≫
        tensorObj.{v₁, u₁} (𝒞 := inst) Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ◁
          η_ X (@rightDual.{v₁, u₁} _ inst inst_1 X _))
      (monoidalComp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) Y
          (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) (@rightDual.{v₁, u₁} _ inst inst_1 Y _) Y)
            (@rightDual.{v₁, u₁} _ inst inst_1 X _)))
        (Y ◁ @rightDual.{v₁, u₁} _ inst inst_1 Y _ ◁ f ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _)
        (monoidalComp (Y := tensorObj.{v₁, u₁} (𝒞 := inst) Y (@rightDual.{v₁, u₁} _ inst inst_1 X _))
          (Y ◁ ε_ Y (@rightDual.{v₁, u₁} _ inst inst_1 Y _) ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _)
          (g ▷ @rightDual.{v₁, u₁} _ inst inst_1 X _ ⊗≫
            𝟙 (tensorObj.{v₁, u₁} (𝒞 := inst) Z (@rightDual.{v₁, u₁} _ inst inst_1 X _))))))
Tactic: calc
    _ = 𝟙 _ ⊗≫ (η_ Y Yᘁ ▷ 𝟙_ C ≫ (Y ⊗ Yᘁ) ◁ η_ X Xᘁ) ⊗≫ Y ◁ Yᘁ ◁ f ▷ Xᘁ ⊗≫
        Y ◁ ε_ Y Yᘁ ▷ Xᘁ ⊗≫ g ▷ Xᘁ ⊗≫ 𝟙 _ := by
      rw [tensorHom_def']; monoidal
    _ = η_ X Xᘁ ⊗≫ (η_ Y Yᘁ ▷ (X ⊗ Xᘁ) ≫ (Y ⊗ Yᘁ) ◁ f ▷ Xᘁ) ⊗≫
        Y ◁ ε_ Y Yᘁ ▷ Xᘁ ⊗≫ g ▷ Xᘁ ⊗≫ 𝟙 _ := by
      rw [← whisker_exchange]; monoidal
    _ = η_ X Xᘁ ⊗≫ f ▷ Xᘁ ⊗≫ (η_ Y Yᘁ ▷ Y ⊗≫ Y ◁ ε_ Y Yᘁ) ▷ Xᘁ ⊗≫ g ▷ Xᘁ ⊗≫ 𝟙 _ := by
      rw [← whisker_exchange]; monoidal
    _ = η_ X Xᘁ ≫ f ▷ Xᘁ ≫ g ▷ Xᘁ := by
      rw [evaluation_coevaluation'']; monoidal