-- Mathlib/CategoryTheory/Monoidal/Bimod.lean
theorem triangle_bimod {X Y Z : Mon_ C} (M : Bimod X Y) (N : Bimod Y Z) :
    (associatorBimod M (regular Y) N).hom ≫ whiskerLeft M (leftUnitorBimod N).hom =
      whiskerRight (rightUnitorBimod M).hom N

Proof state:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
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)
X Y Z : Mon_ C
M : Bimod X Y
N : Bimod Y Z
⊢ ((PreservesCoequalizer.iso (tensorRight N.X) (M.actRight ▷ Y.X) ((α_ M.X Y.X Y.X).hom ≫ M.X ◁ Y.mul)).inv ≫
        coequalizer.desc
          ((α_ M.X Y.X N.X).hom ≫
            M.X ◁ coequalizer.π (Y.mul ▷ N.X) ((α_ Y.X Y.X N.X).hom ≫ Y.X ◁ N.actLeft) ≫
              coequalizer.π (M.actRight ▷ TensorBimod.X (regular Y) N)
                ((α_ M.X Y.X (TensorBimod.X (regular Y) N)).hom ≫ M.X ◁ TensorBimod.actLeft (regular Y) N))
          ⋯) ≫
      colimMap
        (parallelPairHom (M.actRight ▷ TensorBimod.X (regular Y) N)
          ((α_ M.X Y.X (TensorBimod.X (regular Y) N)).hom ≫ M.X ◁ TensorBimod.actLeft (regular Y) N) (M.actRight ▷ N.X)
          ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ((M.X ⊗ Y.X) ◁ LeftUnitorBimod.hom N) (M.X ◁ LeftUnitorBimod.hom N) ⋯
          ⋯) =
    coequalizer.π (TensorBimod.actRight M (regular Y) ▷ N.X)
        ((α_ (TensorBimod.X M (regular Y)) Y.X N.X).hom ≫ TensorBimod.X M (regular Y) ◁ N.actLeft) ≫
      colimMap
        (parallelPairHom (TensorBimod.actRight M (regular Y) ▷ N.X)
          ((α_ (TensorBimod.X M (regular Y)) Y.X N.X).hom ≫ TensorBimod.X M (regular Y) ◁ N.actLeft) (M.actRight ▷ N.X)
          ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) (RightUnitorBimod.hom M ▷ Y.X ▷ N.X) (RightUnitorBimod.hom M ▷ N.X) ⋯
          ⋯)

Tactic:
slice_rhs 1 2 => rw [ι_colimMap, parallelPairHom_app_one]

Result:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
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)
X Y Z : Mon_ C
M : Bimod X Y
N : Bimod Y Z
⊢ ((PreservesCoequalizer.iso (tensorRight N.X) (M.actRight ▷ Y.X) ((α_ M.X Y.X Y.X).hom ≫ M.X ◁ Y.mul)).inv ≫
        coequalizer.desc
          ((α_ M.X Y.X N.X).hom ≫
            M.X ◁ coequalizer.π (Y.mul ▷ N.X) ((α_ Y.X Y.X N.X).hom ≫ Y.X ◁ N.actLeft) ≫
              coequalizer.π (M.actRight ▷ TensorBimod.X (regular Y) N)
                ((α_ M.X Y.X (TensorBimod.X (regular Y) N)).hom ≫ M.X ◁ TensorBimod.actLeft (regular Y) N))
          ⋯) ≫
      colimMap
        (parallelPairHom (M.actRight ▷ TensorBimod.X (regular Y) N)
          ((α_ M.X Y.X (TensorBimod.X (regular Y) N)).hom ≫ M.X ◁ TensorBimod.actLeft (regular Y) N) (M.actRight ▷ N.X)
          ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ((M.X ⊗ Y.X) ◁ LeftUnitorBimod.hom N) (M.X ◁ LeftUnitorBimod.hom N) ⋯
          ⋯) =
    RightUnitorBimod.hom M ▷ N.X ≫
      colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one

Proof state:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
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)
X Y Z : Mon_ C
M : Bimod X Y
N : Bimod Y Z
⊢ (α_ M.X Y.X N.X).hom ≫
      M.X ◁ coequalizer.π (Y.mul ▷ N.X) ((α_ Y.X Y.X N.X).hom ≫ Y.X ◁ N.actLeft) ≫
        M.X ◁ LeftUnitorBimod.hom N ≫
          colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one =
    coequalizer.π (M.actRight ▷ Y.X) ((α_ M.X Y.X Y.X).hom ≫ M.X ◁ Y.mul) ▷ N.X ≫
      coequalizer.desc M.actRight ⋯ ▷ N.X ≫
        colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one

Tactic:
dsimp [LeftUnitorBimod.hom]

Result:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
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)
X Y Z : Mon_ C
M : Bimod X Y
N : Bimod Y Z
⊢ (α_ M.X Y.X N.X).hom ≫
      M.X ◁ coequalizer.π (Y.mul ▷ N.X) ((α_ Y.X Y.X N.X).hom ≫ Y.X ◁ N.actLeft) ≫
        M.X ◁ coequalizer.desc N.actLeft ⋯ ≫
          colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one =
    coequalizer.π (M.actRight ▷ Y.X) ((α_ M.X Y.X Y.X).hom ≫ M.X ◁ Y.mul) ▷ N.X ≫
      coequalizer.desc M.actRight ⋯ ▷ N.X ≫
        colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one

Proof state:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
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)
X Y Z : Mon_ C
M : Bimod X Y
N : Bimod Y Z
⊢ (α_ M.X Y.X N.X).hom ≫
      M.X ◁ N.actLeft ≫
        colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one =
    M.actRight ▷ N.X ≫
      colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one

Tactic:
slice_rhs 1 2 => rw [coequalizer.condition]

Result:
case h.h
C : Type u₁
inst✝⁴ : Category.{v₁, u₁} C
inst✝³ : MonoidalCategory C
inst✝² : HasCoequalizers C
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)
X Y Z : Mon_ C
M : Bimod X Y
N : Bimod Y Z
⊢ (α_ M.X Y.X N.X).hom ≫
      M.X ◁ N.actLeft ≫
        colimit.ι (parallelPair (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one =
    ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft)