-- 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 : Mon.{v₁, u₁} C
M N : Bimod X Y
f : M ⟶ N
⊢ Eq (α :=
    tensorObj.{v₁, u₁} (𝒞 := inst)
        { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯, right_assoc := ⋯,
            middle_assoc := ⋯ : Bimod X X }.X
        M.X ⟶
      ({ X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯, right_assoc := ⋯,
              middle_assoc := ⋯ : Bimod X X }.tensorBimod
          N).X)
    (coequalizer.π
        ({ X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
              right_assoc := ⋯, middle_assoc := ⋯ }.actRight ▷
          M.X)
        (Iso.hom
            (α_
              { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                  right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.X
              X.X M.X) ≫
          { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.X ◁
            M.actLeft) ≫
      Hom.hom
        ({ X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
              right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.whiskerLeft
          f))
    (CategoryStruct.comp (Y :=
      coequalizer
        ({ X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
              right_assoc := ⋯, middle_assoc := ⋯ }.actRight ▷
          M.X)
        (Iso.hom
            (α_
              { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                  right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.X
              X.X M.X) ≫
          { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.X ◁
            M.actLeft))
      (coequalizer.π
        ({ X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
              right_assoc := ⋯, middle_assoc := ⋯ }.actRight ▷
          M.X)
        (Iso.hom
            (α_
              { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                  right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.X
              X.X M.X) ≫
          { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }.X ◁
            M.actLeft))
      (Hom.hom
        (Iso.hom
            (isoOfIso { hom := LeftUnitorBimod.hom M, inv := LeftUnitorBimod.inv M, hom_inv_id := ⋯, inv_hom_id := ⋯ } ⋯
              ⋯) ≫
          f ≫
            Iso.inv
              (isoOfIso
                { hom := LeftUnitorBimod.hom N, inv := LeftUnitorBimod.inv N, hom_inv_id := ⋯, inv_hom_id := ⋯ :
                  TensorBimod.X
                      { X := X.X, actLeft := μ, one_actLeft := ⋯, left_assoc := ⋯, actRight := μ, actRight_one := ⋯,
                        right_assoc := ⋯, middle_assoc := ⋯ : Bimod X X }
                      N ≅
                    N.X }
                ⋯ ⋯))))
have h : Eq (α :=
  tensorObj.{v₁, u₁} (𝒞 := inst) X.X M.X ⟶
    TensorBimod.X
      { X := X.X, actLeft := μ, one_actLeft := sorry, left_assoc := sorry, actRight := μ, actRight_one := sorry, right_assoc := sorry,
        middle_assoc := sorry : Bimod X X }
      N)
  (CategoryStruct.comp (Y := coequalizer (μ ▷ M.X) (Iso.hom (α_ X.X X.X M.X) ≫ X.X ◁ M.actLeft))
      (coequalizer.π (μ ▷ M.X) (Iso.hom (α_ X.X X.X M.X) ≫ X.X ◁ M.actLeft))
      (colimMap
        (parallelPairHom (μ ▷ M.X) (Iso.hom (α_ X.X X.X M.X) ≫ X.X ◁ M.actLeft) (μ ▷ N.X)
          (Iso.hom (α_ X.X X.X N.X) ≫ X.X ◁ N.actLeft) (tensorObj.{v₁, u₁} (𝒞 := inst) X.X X.X ◁ f.hom) (X.X ◁ f.hom) sorry
          sorry)) :
    tensorObj.{v₁, u₁} (𝒞 := inst) X.X M.X ⟶
      TensorBimod.X
        { X := X.X, actLeft := μ, one_actLeft := sorry, left_assoc := sorry, actRight := μ, actRight_one := sorry, right_assoc := sorry,
          middle_assoc := sorry : Bimod X X }
        N)
  (CategoryStruct.comp (Y := coequalizer (μ ▷ M.X) (Iso.hom (α_ X.X X.X M.X) ≫ X.X ◁ M.actLeft))
      (coequalizer.π (μ ▷ M.X) (Iso.hom (α_ X.X X.X M.X) ≫ X.X ◁ M.actLeft) :
        tensorObj.{v₁, u₁} (𝒞 := inst) X.X M.X ⟶ coequalizer (μ ▷ M.X) (Iso.hom (α_ X.X X.X M.X) ≫ X.X ◁ M.actLeft))
      (LeftUnitorBimod.hom M ≫ f.hom ≫ LeftUnitorBimod.inv N :
        TensorBimod.X
            { X := X.X, actLeft := μ, one_actLeft := sorry, left_assoc := sorry, actRight := μ, actRight_one := sorry,
              right_assoc := sorry, middle_assoc := sorry : Bimod X X }
            M ⟶
          TensorBimod.X
            { X := X.X, actLeft := μ, one_actLeft := sorry, left_assoc := sorry, actRight := μ, actRight_one := sorry,
              right_assoc := sorry, middle_assoc := sorry : Bimod X X }
            N) :
    tensorObj.{v₁, u₁} (𝒞 := inst) X.X M.X ⟶
      TensorBimod.X
        { X := X.X, actLeft := μ, one_actLeft := sorry, left_assoc := sorry, actRight := μ, actRight_one := sorry, right_assoc := sorry,
          middle_assoc := sorry : Bimod X X }
        N)
Tactic: dsimp