-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Bimod (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
inst_1 : MonoidalCategory.{v₁, u₁} C
inst_2 : HasCoequalizers.{v₁, u₁} C
R S T : Mon.{v₁, u₁} C
P : Bimod R S
Q : Bimod S T
inst_3 : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
⊢ Eq (α :=
    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
      coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
    (Iso.inv (α_ (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) P.X Q.X) ≫
        (Iso.hom (α_ R.X R.X P.X) ▷ Q.X ≫ (R.X ◁ P.actLeft) ▷ Q.X ≫ P.actLeft ▷ Q.X) ≫
          coequalizer.π (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft) :
      tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
        coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
    (CategoryStruct.comp (Y :=
        tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X)
          (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)))
        (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X ◁
            coequalizer.π (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft) :
          tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X)
              (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
            tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X)
              (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)))
        (Iso.hom (α_ R.X R.X (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))) ≫
            CategoryStruct.comp (Y :=
              tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)))
              (R.X ◁ actLeft P Q :
                tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                    (tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                      (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))) ⟶
                  tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                    (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)))
              (actLeft P Q : tensorObj.{v₁, u₁} (𝒞 := inst) R.X (X P Q) ⟶ X P Q) :
          tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X)
              (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)) ⟶
            coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)) :
      tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
        coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
have slice_rhs : Eq (α :=
  tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
    coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
  (Iso.inv (α_ (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) P.X Q.X) ≫
      (Iso.hom (α_ R.X R.X P.X) ▷ Q.X ≫ (R.X ◁ P.actLeft) ▷ Q.X ≫ P.actLeft ▷ Q.X) ≫
        coequalizer.π (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft) :
    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
      coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
  (CategoryStruct.comp (Y :=
      tensorObj.{v₁, u₁} (𝒞 := inst) R.X (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)))
      (CategoryStruct.comp (Y :=
          tensorObj.{v₁, u₁} (𝒞 := inst) R.X
            (tensorObj.{v₁, u₁} (𝒞 := inst) R.X
              (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))))
          (Iso.hom (α_ R.X R.X (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) ≫
              R.X ◁ R.X ◁ coequalizer.π (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft) :
            tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X)
                (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
              tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                (tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                  (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))))
          (R.X ◁ actLeft P Q :
            tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                (tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                  (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))) ⟶
              tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))) :
        tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X)
            (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
          tensorObj.{v₁, u₁} (𝒞 := inst) R.X
            (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)))
      (actLeft P Q : tensorObj.{v₁, u₁} (𝒞 := inst) R.X (X P Q) ⟶ X P Q) :
    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X R.X) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) ⟶
      coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
Tactic: slice_rhs 1 2 => rw [associator_naturality_right]