-- 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)
inst_4 : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ Eq (α :=
    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.X ⟶
      coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
    (Iso.inv (α_ R.X P.X Q.X) ▷ T.X ≫
        ((Iso.hom (α_ (tensorObj.{v₁, u₁} (𝒞 := inst) R.X P.X) Q.X T.X) ≫
              P.actLeft ▷ tensorObj.{v₁, u₁} (𝒞 := inst) Q.X T.X) ≫
            P.X ◁ Q.actRight) ≫
          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 (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.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)
                (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)) T.X))
            (Iso.hom (α_ R.X (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) T.X) ≫
                R.X ◁ coequalizer.π (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft) ▷ T.X :
              tensorObj.{v₁, u₁} (𝒞 := inst)
                  (tensorObj.{v₁, u₁} (𝒞 := inst) R.X (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.X ⟶
                tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                  (tensorObj.{v₁, u₁} (𝒞 := inst)
                    (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)) T.X))
            (R.X ◁ actRight P Q :
              tensorObj.{v₁, u₁} (𝒞 := inst) R.X
                  (tensorObj.{v₁, u₁} (𝒞 := inst)
                    (coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft)) T.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) (tensorObj.{v₁, u₁} (𝒞 := inst) R.X (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X))
              T.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 (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.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 (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.X ⟶
    coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
  (Iso.inv (α_ R.X P.X Q.X) ▷ T.X ≫
      ((Iso.hom (α_ (tensorObj.{v₁, u₁} (𝒞 := inst) R.X P.X) Q.X T.X) ≫
            P.actLeft ▷ tensorObj.{v₁, u₁} (𝒞 := inst) Q.X T.X) ≫
          P.X ◁ Q.actRight) ≫
        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 (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.X ⟶
      coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
  (Iso.hom (α_ R.X (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) T.X) ≫
      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 ◁ Iso.hom (α_ P.X Q.X T.X) ≫
            R.X ◁ P.X ◁ Q.actRight ≫
              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) R.X
              (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X) T.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 (tensorObj.{v₁, u₁} (𝒞 := inst) P.X Q.X)) T.X ⟶
      coequalizer (P.actRight ▷ Q.X) (Iso.hom (α_ P.X S.X Q.X) ≫ P.X ◁ Q.actLeft))
Tactic: slice_rhs 2 3 => rw [← whiskerLeft_comp, π_tensor_id_actRight,
    whiskerLeft_comp, whiskerLeft_comp]