-- 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
⊢ 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

Tactic:
dsimp

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 (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) ≫
      AssociatorBimod.hom M (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) ⋯
          ⋯)

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 =
    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:
slice_rhs 1 2 => rw [← comp_whiskerRight, coequalizer.π_desc]

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 =
    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:
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
⊢ (M.associatorBimod (regular Y) N).hom ≫ M.whiskerLeft N.leftUnitorBimod.hom = whiskerRight M.rightUnitorBimod.hom N

Tactic:
dsimp [associatorBimod, leftUnitorBimod, rightUnitorBimod]

Result:
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 =
    whiskerRight
      (isoOfIso { hom := RightUnitorBimod.hom M, inv := RightUnitorBimod.inv M, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
          ⋯).hom
      N

Proof state:
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 =
    whiskerRight
      (isoOfIso { hom := RightUnitorBimod.hom M, inv := RightUnitorBimod.inv M, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
          ⋯).hom
      N

Tactic:
ext

Result:
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