-- 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 :=
      coequalizer (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))
      (coequalizer.π (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))
      (coequalizer.desc (AssociatorBimod.homAux M (regular Y) 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) ⋯ ⋯)))
    (CategoryStruct.comp (Y :=
      coequalizer (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))
      (coequalizer.π (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))
      (colimMap
        (parallelPairHom (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)
          (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft) (RightUnitorBimod.hom M ▷ Y.X ▷ N.X)
          (RightUnitorBimod.hom M ▷ N.X) ⋯ ⋯)))
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)) (AssociatorBimod.homAux M (regular Y) 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)) :
    tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶ TensorBimod.X M N)
  (CategoryStruct.comp (Y :=
      coequalizer (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))
      (coequalizer.π (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))
      (colimMap
        (parallelPairHom (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)
          (M.actRight ▷ N.X) (Iso.hom (α_ M.X Y.X N.X) ≫ M.X ◁ N.actLeft) (RightUnitorBimod.hom M ▷ Y.X ▷ N.X)
          (RightUnitorBimod.hom M ▷ N.X) sorry sorry)) :
    tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M (regular Y)) N.X ⟶ TensorBimod.X M N)
Tactic: slice_lhs 1 2 => rw [coequalizer.π_desc]