-- Lean 4 proof step from Mathlib.CategoryTheory.Monoidal.Bimod (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
inst_1 : MonoidalCategory.{v₁, u₁} C
inst_2 : HasCoequalizers.{v₁, u₁} C
inst_3 : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorLeft X)
inst_4 : ∀ (X : C), PreservesColimitsOfSize.{0, 0, v₁, v₁, u₁, u₁} (tensorRight X)
X Y Z : Mon.{v₁, u₁} C
M : Bimod X Y
N : Bimod Y Z
⊢ Eq (α := tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶ TensorBimod.X M N)
    (CategoryStruct.comp (Y := TensorBimod.X M ((regular Y).tensorBimod N))
        (CategoryStruct.comp (Y :=
            coequalizer (X :=
              tensorObj.{v₁, u₁} (𝒞 := inst)
                (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X) Y.X) N.X)
              (M.actRight ▷ Y.X ▷ N.X) ((Iso.hom (α_ M.X Y.X Y.X) ≫ M.X ◁ μ) ▷ N.X))
            (Iso.inv
              (PreservesCoequalizer.iso (tensorRight N.X) (M.actRight ▷ Y.X) (Iso.hom (α_ M.X Y.X Y.X) ≫ M.X ◁ μ)))
            (coequalizer.desc
              (Iso.hom (α_ M.X Y.X N.X) ≫
                CategoryStruct.comp (Y :=
                  tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                    (coequalizer (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft)))
                  (M.X ◁ coequalizer.π (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft) :
                    tensorObj.{v₁, u₁} (𝒞 := inst) M.X (tensorObj.{v₁, u₁} (𝒞 := inst) Y.X N.X) ⟶
                      tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                        (coequalizer (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft)))
                  (coequalizer.π (M.actRight ▷ TensorBimod.X (regular Y) N)
                      (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N) :
                    tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                        (coequalizer (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft)) ⟶
                      coequalizer (X :=
                        tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X)
                          (TensorBimod.X (regular Y) N))
                        (M.actRight ▷ TensorBimod.X (regular Y) N)
                        (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N)))
              ⋯) :
          tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶
            TensorBimod.X M ((regular Y).tensorBimod N))
        (colimMap
            (parallelPairHom (M.actRight ▷ TensorBimod.X (regular Y) N)
              (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N)
              (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft)
              (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X ◁ LeftUnitorBimod.hom N) (M.X ◁ LeftUnitorBimod.hom N) ⋯ ⋯) :
          colimit
              (parallelPair (X :=
                tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X) (TensorBimod.X (regular Y) N))
                (M.actRight ▷ TensorBimod.X (regular Y) N)
                (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N)) ⟶
            colimit (parallelPair (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft))) :
      tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶ TensorBimod.X M N)
    (RightUnitorBimod.hom M ▷ N.X ≫
        colimit.ι (parallelPair (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft))
          WalkingParallelPair.one :
      (parallelPair (TensorBimod.actRight M (regular Y) ▷ N.X)
              (Iso.hom (α_ (TensorBimod.X M (regular Y)) Y.X N.X) ≫ TensorBimod.X M (regular Y) ◁ N.actLeft)).obj
          WalkingParallelPair.one ⟶
        colimit (parallelPair (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft)))
have h : Eq (α := tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶ TensorBimod.X M N)
  (CategoryStruct.comp (Y := TensorBimod.X M ((regular Y).tensorBimod N))
    (CategoryStruct.comp (Y :=
        coequalizer (X :=
          tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X) Y.X)
            N.X)
          (M.actRight ▷ Y.X ▷ N.X) ((Iso.hom (α_ M.X Y.X Y.X) ≫ M.X ◁ μ) ▷ N.X))
        (Iso.inv (PreservesCoequalizer.iso (tensorRight N.X) (M.actRight ▷ Y.X) (Iso.hom (α_ M.X Y.X Y.X) ≫ M.X ◁ μ)))
        (coequalizer.desc
          (Iso.hom (α_ M.X Y.X N.X) ≫
            CategoryStruct.comp (Y :=
              tensorObj.{v₁, u₁} (𝒞 := inst) M.X (coequalizer (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft)))
              (M.X ◁ coequalizer.π (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft) :
                tensorObj.{v₁, u₁} (𝒞 := inst) M.X (tensorObj.{v₁, u₁} (𝒞 := inst) Y.X N.X) ⟶
                  tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                    (coequalizer (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft)))
              (coequalizer.π (M.actRight ▷ TensorBimod.X (regular Y) N)
                  (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N) :
                tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                    (coequalizer (μ ▷ N.X) (Iso.hom (α_ Y.X Y.X N.X) ≫ Y.X ◁ N.actLeft)) ⟶
                  coequalizer (X :=
                    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X)
                      (TensorBimod.X (regular Y) N))
                    (M.actRight ▷ TensorBimod.X (regular Y) N)
                    (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N)))
          sorry) :
      tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶ TensorBimod.X M ((regular Y).tensorBimod N))
    (colimMap
        (parallelPairHom (M.actRight ▷ TensorBimod.X (regular Y) N)
          (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N)
          (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft)
          (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X ◁ LeftUnitorBimod.hom N) (M.X ◁ LeftUnitorBimod.hom N) sorry sorry) :
      colimit
          (parallelPair (X :=
            tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X Y.X) (TensorBimod.X (regular Y) N))
            (M.actRight ▷ TensorBimod.X (regular Y) N)
            (Iso.hom (α_ M.X Y.X (TensorBimod.X (regular Y) N)) ≫ M.X ◁ TensorBimod.actLeft (regular Y) N)) ⟶
        colimit (parallelPair (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft))))
  (coequalizer.desc M.actRight sorry ▷ N.X ≫
    colimit.ι (parallelPair (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft)) WalkingParallelPair.one)
Tactic: dsimp [RightUnitorBimod.hom]