-- Mathlib/CategoryTheory/Monoidal/Bimod.lean
theorem middle_assoc' :
    (actLeft P Q ▷ T.X) ≫ actRight P Q =
      (α_ R.X _ T.X).hom ≫ (R.X ◁ actRight P Q) ≫ actLeft P Q

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ ((α_ R.X P.X Q.X).inv ▷ T.X ≫
        P.actLeft ▷ Q.X ▷ T.X ≫ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) ▷ T.X) ≫
      actRight P Q =
    (R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ▷ T.X ≫
      (α_ R.X (coequalizer (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) T.X).hom ≫
        R.X ◁ actRight P Q ≫ actLeft P Q

Tactic:
slice_lhs 3 4 => rw [π_tensor_id_actRight]

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      P.actLeft ▷ Q.X ▷ T.X ≫
        (α_ P.X Q.X T.X).hom ≫
          P.X ◁ Q.actRight ≫ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ▷ T.X ≫
      (α_ R.X (coequalizer (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) T.X).hom ≫
        R.X ◁ actRight P Q ≫ actLeft P Q

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      (((α_ (R.X ⊗ P.X) Q.X T.X).hom ≫ P.actLeft ▷ (Q.X ⊗ T.X)) ≫ P.X ◁ Q.actRight) ≫
        coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (((α_ R.X (P.X ⊗ Q.X) T.X).hom ≫
          R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) ▷ T.X) ≫
        R.X ◁ actRight P Q) ≫
      actLeft P Q

Tactic:
slice_rhs 2 3 => rw [← MonoidalCategory.whiskerLeft_comp, π_tensor_id_actRight,
  MonoidalCategory.whiskerLeft_comp, MonoidalCategory.whiskerLeft_comp]

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      (((α_ (R.X ⊗ P.X) Q.X T.X).hom ≫ P.actLeft ▷ (Q.X ⊗ T.X)) ≫ P.X ◁ Q.actRight) ≫
        coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (α_ R.X (P.X ⊗ Q.X) T.X).hom ≫
      (R.X ◁ (α_ P.X Q.X T.X).hom ≫
          R.X ◁ P.X ◁ Q.actRight ≫ R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ≫
        actLeft P Q

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      (((α_ (R.X ⊗ P.X) Q.X T.X).hom ≫ P.actLeft ▷ (Q.X ⊗ T.X)) ≫ P.X ◁ Q.actRight) ≫
        coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (α_ R.X (P.X ⊗ Q.X) T.X).hom ≫
      R.X ◁ (α_ P.X Q.X T.X).hom ≫
        (((α_ R.X P.X (Q.X ⊗ T.X)).inv ≫ (R.X ⊗ P.X) ◁ Q.actRight) ≫ P.actLeft ▷ Q.X) ≫
          coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)

Tactic:
slice_rhs 4 5 => rw [whisker_exchange]

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      (((α_ (R.X ⊗ P.X) Q.X T.X).hom ≫ P.actLeft ▷ (Q.X ⊗ T.X)) ≫ P.X ◁ Q.actRight) ≫
        coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (α_ R.X (P.X ⊗ Q.X) T.X).hom ≫
      R.X ◁ (α_ P.X Q.X T.X).hom ≫
        (α_ R.X P.X (Q.X ⊗ T.X)).inv ≫
          (P.actLeft ▷ (Q.X ⊗ T.X) ≫ P.X ◁ Q.actRight) ≫
            coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (tensorLeft R.X ⋙ tensorRight T.X).map (coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ≫
      actLeft P Q ▷ T.X ≫ actRight P Q =
    (tensorLeft R.X ⋙ tensorRight T.X).map (coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ≫
      (α_ R.X (X P Q) T.X).hom ≫ R.X ◁ actRight P Q ≫ actLeft P Q

Tactic:
dsimp [X]

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ▷ T.X ≫
      actLeft P Q ▷ T.X ≫ actRight P Q =
    (R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ▷ T.X ≫
      (α_ R.X (coequalizer (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) T.X).hom ≫
        R.X ◁ actRight P Q ≫ actLeft P Q

Proof state:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      (((α_ (R.X ⊗ P.X) Q.X T.X).hom ≫ P.actLeft ▷ (Q.X ⊗ T.X)) ≫ P.X ◁ Q.actRight) ≫
        coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (α_ R.X (P.X ⊗ Q.X) T.X).hom ≫
      (R.X ◁ (α_ P.X Q.X T.X).hom ≫
          R.X ◁ P.X ◁ Q.actRight ≫ R.X ◁ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)) ≫
        actLeft P Q

Tactic:
slice_rhs 4 5 => rw [whiskerLeft_π_actLeft]

Result:
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
R S T : Mon_ C
P : Bimod R S
Q : Bimod S T
inst✝¹ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst✝ : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
⊢ (α_ R.X P.X Q.X).inv ▷ T.X ≫
      (((α_ (R.X ⊗ P.X) Q.X T.X).hom ≫ P.actLeft ▷ (Q.X ⊗ T.X)) ≫ P.X ◁ Q.actRight) ≫
        coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft) =
    (α_ R.X (P.X ⊗ Q.X) T.X).hom ≫
      R.X ◁ (α_ P.X Q.X T.X).hom ≫
        R.X ◁ P.X ◁ Q.actRight ≫
          (α_ R.X P.X Q.X).inv ≫
            P.actLeft ▷ Q.X ≫ coequalizer.π (P.actRight ▷ Q.X) ((α_ P.X S.X Q.X).hom ≫ P.X ◁ Q.actLeft)