-- 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)))
    ((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 :=
        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 })
            (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) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.X) 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))) :
          Functor.obj.{v₁, v₁, u₁, u₁} (tensorRight P.X) (tensorObj.{v₁, u₁} (𝒞 := inst) M.X N.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)))
  (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))
        (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 })
          (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) ≫
              colimit.ι
                (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))
                WalkingParallelPair.one :
            (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)).obj
                WalkingParallelPair.one ⟶
              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)))
          (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_rhs 3 4 => rw [ι_colimMap, parallelPairHom_app_one]