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