-- 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 := (M.tensorBimod N).tensorBimod P ⟶ (M.tensorBimod N').tensorBimod P) (whiskerRight (M.whiskerLeft f) P)
    (Iso.hom (M.associatorBimod N P) ≫ M.whiskerLeft (whiskerRight f P) ≫ Iso.inv (M.associatorBimod N' P))
have dsimp : Eq (α :=
  {
      X :=
        TensorBimod.X
          { X := TensorBimod.X M N, actLeft := TensorBimod.actLeft M N, one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight M N, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry : Bimod W Y }
          P,
      actLeft :=
        TensorBimod.actLeft
          { X := TensorBimod.X M N, actLeft := TensorBimod.actLeft M N, one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight M N, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry : Bimod W Y }
          P,
      one_actLeft := sorry, left_assoc := sorry,
      actRight :=
        TensorBimod.actRight
          { X := TensorBimod.X M N, actLeft := TensorBimod.actLeft M N, one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight M N, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry : Bimod W Y }
          P,
      actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry : Bimod W Z } ⟶
    {
      X :=
        TensorBimod.X
          { X := TensorBimod.X M N', actLeft := TensorBimod.actLeft M N', one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight M N', actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry : Bimod W Y }
          P,
      actLeft :=
        TensorBimod.actLeft
          { X := TensorBimod.X M N', actLeft := TensorBimod.actLeft M N', one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight M N', actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry }
          P,
      one_actLeft := sorry, left_assoc := sorry,
      actRight :=
        TensorBimod.actRight
          { X := TensorBimod.X M N', actLeft := TensorBimod.actLeft M N', one_actLeft := sorry, left_assoc := sorry,
            actRight := TensorBimod.actRight M N', actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry }
          P,
      actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry })
  (whiskerRight (M.whiskerLeft f) P)
  (Iso.hom
      (isoOfIso
        { hom := AssociatorBimod.hom M N P, inv := AssociatorBimod.inv M N P, hom_inv_id := sorry, inv_hom_id := sorry :
          TensorBimod.X
              { X := TensorBimod.X M N, actLeft := TensorBimod.actLeft M N, one_actLeft := sorry, left_assoc := sorry,
                actRight := TensorBimod.actRight M N, actRight_one := sorry, right_assoc := sorry, middle_assoc := sorry :
                Bimod W Y }
              P ≅
            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 } }
        sorry sorry) ≫
    M.whiskerLeft (whiskerRight f P) ≫
      Iso.inv
        (isoOfIso
          { hom := AssociatorBimod.hom M N' P, inv := AssociatorBimod.inv M N' P, hom_inv_id := sorry, inv_hom_id := sorry } sorry
          sorry))
Tactic: dsimp [tensorHom, tensorBimod, associatorBimod]