-- 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
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
⊢ ((isoOfIso
            { hom := AssociatorBimod.hom M (regular Y) N, inv := AssociatorBimod.inv M (regular Y) N, hom_inv_id := ⋯,
              inv_hom_id := ⋯ }
            ⋯ ⋯).hom ≫
        M.whiskerLeft
          (isoOfIso { hom := LeftUnitorBimod.hom N, inv := LeftUnitorBimod.inv N, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
              ⋯).hom).hom =
    (whiskerRight
        (isoOfIso { hom := RightUnitorBimod.hom M, inv := RightUnitorBimod.inv M, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
            ⋯).hom
        N).hom

Tactic:
apply coequalizer.hom_ext

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
⊢ coequalizer.π ((M.tensorBimod (regular Y)).actRight ▷ N.X)
        ((α_ (M.tensorBimod (regular Y)).X Y.X N.X).hom ≫ (M.tensorBimod (regular Y)).X ◁ N.actLeft) ≫
      ((isoOfIso
              { hom := AssociatorBimod.hom M (regular Y) N, inv := AssociatorBimod.inv M (regular Y) N, hom_inv_id := ⋯,
                inv_hom_id := ⋯ }
              ⋯ ⋯).hom ≫
          M.whiskerLeft
            (isoOfIso { hom := LeftUnitorBimod.hom N, inv := LeftUnitorBimod.inv N, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
                ⋯).hom).hom =
    coequalizer.π ((M.tensorBimod (regular Y)).actRight ▷ N.X)
        ((α_ (M.tensorBimod (regular Y)).X Y.X N.X).hom ≫ (M.tensorBimod (regular Y)).X ◁ N.actLeft) ≫
      (whiskerRight
          (isoOfIso { hom := RightUnitorBimod.hom M, inv := RightUnitorBimod.inv M, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
              ⋯).hom
          N).hom

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.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:
refine (cancel_epi ((tensorRight _).map (coequalizer.π _ _))).1 ?_

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
⊢ (tensorRight N.X).map
        (coequalizer.π (M.actRight ▷ (regular Y).X) ((α_ M.X Y.X (regular Y).X).hom ≫ M.X ◁ (regular Y).actLeft)) ≫
      ((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) ⋯ ⋯) =
    (tensorRight N.X).map
        (coequalizer.π (M.actRight ▷ (regular Y).X) ((α_ M.X Y.X (regular Y).X).hom ≫ M.X ◁ (regular Y).actLeft)) ≫
      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