-- Mathlib/CategoryTheory/Monoidal/Bimod.lean
theorem whiskerLeft_id_bimod {X Y Z : Mon_ C} {M : Bimod X Y} {N : Bimod Y Z} :
    whiskerLeft M (𝟙 N) = 𝟙 (M.tensorBimod 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.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ (M.whiskerLeft (𝟙 N)).hom =
    coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ (𝟙 (M.tensorBimod N)).hom

Tactic:
dsimp only [tensorBimod_X, whiskerLeft_hom, id_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
⊢ coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫
      colimMap
        (parallelPairHom (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) (M.actRight ▷ N.X)
          ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ((M.X ⊗ Y.X) ◁ 𝟙 N.X) (M.X ◁ 𝟙 N.X) ⋯ ⋯) =
    coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ 𝟙 (TensorBimod.X M 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
⊢ M.whiskerLeft (𝟙 N) = 𝟙 (M.tensorBimod 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
⊢ (M.whiskerLeft (𝟙 N)).hom = (𝟙 (M.tensorBimod 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
⊢ coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫
      colimMap
        (parallelPairHom (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) (M.actRight ▷ N.X)
          ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ((M.X ⊗ Y.X) ◁ 𝟙 N.X) (M.X ◁ 𝟙 N.X) ⋯ ⋯) =
    coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ 𝟙 (TensorBimod.X M N)

Tactic:
simp only [MonoidalCategory.whiskerLeft_id, ι_colimMap, parallelPair_obj_one,
  parallelPairHom_app_one, Category.id_comp]

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
⊢ 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 ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ 𝟙 (TensorBimod.X M 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
⊢ (M.whiskerLeft (𝟙 N)).hom = (𝟙 (M.tensorBimod N)).hom

Tactic:
apply Limits.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.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ (M.whiskerLeft (𝟙 N)).hom =
    coequalizer.π (M.actRight ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ (𝟙 (M.tensorBimod 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
⊢ 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 ▷ N.X) ((α_ M.X Y.X N.X).hom ≫ M.X ◁ N.actLeft) ≫ 𝟙 (TensorBimod.X M N)

Tactic:
erw [Category.comp_id]

Result:
no goals