-- 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)
W X Y Z : Mon.{v₁, u₁} C
M : Bimod W X
N N' : Bimod X Y
f : N ⟶ N'
P : Bimod Y Z
⊢ Eq (α :=
    Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) ⟶
      colimit
        (parallelPair (TensorBimod.actRight M N' ▷ P.X)
          (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft)))
    (CategoryStruct.comp (Y :=
        Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X)
          (coequalizer (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft)))
        (coequalizer.π (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft) ▷ P.X)
        (colimMap
              (parallelPairHom (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft) (M.actRight ▷ N'.X)
                (Iso.hom (α_ M.X X.X N'.X) ≫ M.X ◁ N'.actLeft) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X ◁ f.hom)
                (M.X ◁ f.hom) ⋯ ⋯) ▷
            P.X ≫
          colimit.ι
            (parallelPair (TensorBimod.actRight M N' ▷ P.X)
              (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft))
            WalkingParallelPair.one) :
      Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) ⟶
        colimit
          (parallelPair (TensorBimod.actRight M N' ▷ P.X)
            (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft)))
    (CategoryStruct.comp (Y :=
        Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X)
          (coequalizer (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft)))
        (coequalizer.π (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft) ▷ P.X)
        (CategoryStruct.comp (Y :=
          TensorBimod.X M
            { X := TensorBimod.X N' P, actLeft := TensorBimod.actLeft N' P, one_actLeft := ⋯, left_assoc := ⋯,
              actRight := TensorBimod.actRight N' P, actRight_one := ⋯, right_assoc := ⋯, middle_assoc := ⋯ :
              Bimod X Z })
          (CategoryStruct.comp (Y :=
              TensorBimod.X M
                { X := TensorBimod.X N P, actLeft := TensorBimod.actLeft N P, one_actLeft := ⋯, left_assoc := ⋯,
                  actRight := TensorBimod.actRight N P, actRight_one := ⋯, right_assoc := ⋯, middle_assoc := ⋯ :
                  Bimod X Z })
              (CategoryStruct.comp (Y :=
                  coequalizer (X :=
                    tensorObj.{v₁, u₁} (𝒞 := inst)
                      (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) N.X) P.X)
                    (M.actRight ▷ N.X ▷ P.X) ((Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft) ▷ P.X))
                  (Iso.inv
                    (PreservesCoequalizer.iso (tensorRight P.X) (M.actRight ▷ N.X)
                      (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft)))
                  (coequalizer.desc
                    (Iso.hom (α_ M.X N.X P.X) ≫
                      CategoryStruct.comp (Y :=
                        tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                          (coequalizer (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)))
                        (M.X ◁ coequalizer.π (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft) :
                          tensorObj.{v₁, u₁} (𝒞 := inst) M.X (tensorObj.{v₁, u₁} (𝒞 := inst) N.X P.X) ⟶
                            tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                              (coequalizer (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)))
                        (coequalizer.π (M.actRight ▷ TensorBimod.X N P)
                            (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P) :
                          tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                              (coequalizer (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)) ⟶
                            coequalizer (X :=
                              tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X)
                                (TensorBimod.X N P))
                              (M.actRight ▷ TensorBimod.X N P)
                              (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P)))
                    ⋯) :
                tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M N) P.X ⟶ TensorBimod.X M (N.tensorBimod P))
              (colimMap
                  (parallelPairHom (M.actRight ▷ TensorBimod.X N P)
                    (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P)
                    (M.actRight ▷ TensorBimod.X N' P)
                    (Iso.hom (α_ M.X X.X (TensorBimod.X N' P)) ≫ M.X ◁ TensorBimod.actLeft N' P)
                    (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X ◁
                      colimMap
                        (parallelPairHom (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)
                          (N'.actRight ▷ P.X) (Iso.hom (α_ N'.X Y.X P.X) ≫ N'.X ◁ P.actLeft) (f.hom ▷ Y.X ▷ P.X)
                          (f.hom ▷ P.X) ⋯ ⋯))
                    (M.X ◁
                      colimMap
                        (parallelPairHom (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)
                          (N'.actRight ▷ P.X) (Iso.hom (α_ N'.X Y.X P.X) ≫ N'.X ◁ P.actLeft) (f.hom ▷ Y.X ▷ P.X)
                          (f.hom ▷ P.X) ⋯ ⋯))
                    ⋯ ⋯) :
                colimit
                    (parallelPair (X :=
                      tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) (TensorBimod.X N P))
                      (M.actRight ▷ TensorBimod.X N P)
                      (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P)) ⟶
                  colimit
                    (parallelPair (X :=
                      tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) (TensorBimod.X N' P))
                      (M.actRight ▷ TensorBimod.X N' P)
                      (Iso.hom (α_ M.X X.X (TensorBimod.X N' P)) ≫ M.X ◁ TensorBimod.actLeft N' P))) :
            tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M N) P.X ⟶
              TensorBimod.X M
                { X := TensorBimod.X N' P, actLeft := TensorBimod.actLeft N' P, one_actLeft := ⋯, left_assoc := ⋯,
                  actRight := TensorBimod.actRight N' P, actRight_one := ⋯, right_assoc := ⋯, middle_assoc := ⋯ :
                  Bimod X Z })
          (AssociatorBimod.inv M N' P : (M.tensorBimod (N'.tensorBimod P)).X ⟶ ((M.tensorBimod N').tensorBimod P).X)) :
      Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) ⟶
        colimit
          (parallelPair (TensorBimod.actRight M N' ▷ P.X)
            (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft)))
have h : Eq (α :=
  Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) ⟶
    colimit
      (parallelPair (TensorBimod.actRight M N' ▷ P.X)
        (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft)))
  ((M.X ◁ f.hom ≫
          colimit.ι (parallelPair (M.actRight ▷ N'.X) (Iso.hom (α_ M.X X.X N'.X) ≫ M.X ◁ N'.actLeft))
            WalkingParallelPair.one) ▷
        P.X ≫
      colimit.ι
        (parallelPair (TensorBimod.actRight M N' ▷ P.X)
          (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft))
        WalkingParallelPair.one :
    Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) ⟶
      colimit
        (parallelPair (TensorBimod.actRight M N' ▷ P.X)
          (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft)))
  (CategoryStruct.comp (Y :=
      Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X)
        (coequalizer (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft)))
      (coequalizer.π (M.actRight ▷ N.X) (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft) ▷ P.X)
      (CategoryStruct.comp (Y :=
        TensorBimod.X M
          { X := TensorBimod.X N' P, actLeft := TensorBimod.actLeft N' P, one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight N' P, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry : Bimod X Z })
        (CategoryStruct.comp (Y :=
            TensorBimod.X M
              { X := TensorBimod.X N P, actLeft := TensorBimod.actLeft N P, one_actLeft := sorry, left_assoc := sorry,
                actRight := TensorBimod.actRight N P, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry :
                Bimod X Z })
            (CategoryStruct.comp (Y :=
                coequalizer (X :=
                  tensorObj.{v₁, u₁} (𝒞 := inst)
                    (tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) N.X) P.X)
                  (M.actRight ▷ N.X ▷ P.X) ((Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft) ▷ P.X))
                (Iso.inv
                  (PreservesCoequalizer.iso (tensorRight P.X) (M.actRight ▷ N.X)
                    (Iso.hom (α_ M.X X.X N.X) ≫ M.X ◁ N.actLeft)))
                (coequalizer.desc
                  (Iso.hom (α_ M.X N.X P.X) ≫
                    CategoryStruct.comp (Y :=
                      tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                        (coequalizer (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)))
                      (M.X ◁ coequalizer.π (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft) :
                        tensorObj.{v₁, u₁} (𝒞 := inst) M.X (tensorObj.{v₁, u₁} (𝒞 := inst) N.X P.X) ⟶
                          tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                            (coequalizer (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)))
                      (coequalizer.π (M.actRight ▷ TensorBimod.X N P)
                          (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P) :
                        tensorObj.{v₁, u₁} (𝒞 := inst) M.X
                            (coequalizer (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)) ⟶
                          coequalizer (X :=
                            tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) (TensorBimod.X N P))
                            (M.actRight ▷ TensorBimod.X N P)
                            (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P)))
                  sorry) :
              tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M N) P.X ⟶ TensorBimod.X M (N.tensorBimod P))
            (colimMap
                (parallelPairHom (M.actRight ▷ TensorBimod.X N P)
                  (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P)
                  (M.actRight ▷ TensorBimod.X N' P)
                  (Iso.hom (α_ M.X X.X (TensorBimod.X N' P)) ≫ M.X ◁ TensorBimod.actLeft N' P)
                  (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X ◁
                    colimMap
                      (parallelPairHom (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)
                        (N'.actRight ▷ P.X) (Iso.hom (α_ N'.X Y.X P.X) ≫ N'.X ◁ P.actLeft) (f.hom ▷ Y.X ▷ P.X)
                        (f.hom ▷ P.X) sorry sorry))
                  (M.X ◁
                    colimMap
                      (parallelPairHom (N.actRight ▷ P.X) (Iso.hom (α_ N.X Y.X P.X) ≫ N.X ◁ P.actLeft)
                        (N'.actRight ▷ P.X) (Iso.hom (α_ N'.X Y.X P.X) ≫ N'.X ◁ P.actLeft) (f.hom ▷ Y.X ▷ P.X)
                        (f.hom ▷ P.X) sorry sorry))
                  sorry sorry) :
              colimit
                  (parallelPair (X :=
                    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) (TensorBimod.X N P))
                    (M.actRight ▷ TensorBimod.X N P)
                    (Iso.hom (α_ M.X X.X (TensorBimod.X N P)) ≫ M.X ◁ TensorBimod.actLeft N P)) ⟶
                colimit
                  (parallelPair (X :=
                    tensorObj.{v₁, u₁} (𝒞 := inst) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X X.X) (TensorBimod.X N' P))
                    (M.actRight ▷ TensorBimod.X N' P)
                    (Iso.hom (α_ M.X X.X (TensorBimod.X N' P)) ≫ M.X ◁ TensorBimod.actLeft N' P))) :
          tensorObj.{v₁, u₁} (𝒞 := inst) (TensorBimod.X M N) P.X ⟶
            TensorBimod.X M
              { X := TensorBimod.X N' P, actLeft := TensorBimod.actLeft N' P, one_actLeft := sorry, left_assoc := sorry,
                actRight := TensorBimod.actRight N' P, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry :
                Bimod X Z })
        (AssociatorBimod.inv M N' P : (M.tensorBimod (N'.tensorBimod P)).X ⟶ ((M.tensorBimod N').tensorBimod P).X)) :
    Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) ⟶
      colimit
        (parallelPair (TensorBimod.actRight M N' ▷ P.X)
          (Iso.hom (α_ (TensorBimod.X M N') Y.X P.X) ≫ TensorBimod.X M N' ◁ P.actLeft)))
Tactic: slice_lhs 1 2 => rw [← comp_whiskerRight, ι_colimMap, parallelPairHom_app_one]