∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {E : Type u₃} [inst_2 : CategoryTheory.Category.{v₃, u₃} E] {E' : Type u₄}
  [inst_3 : CategoryTheory.Category.{v₄, u₄} E'] (F : CategoryTheory.Functor C D) (G : CategoryTheory.Functor D E)
  (H : CategoryTheory.Functor E E'),
  (F.associator G H).core =
    (F.comp G).coreComp H ≪≫
      CategoryTheory.Functor.isoWhiskerRight (F.coreComp G) H.core ≪≫
        F.core.associator G.core H.core ≪≫ (F.core.isoWhiskerLeft (G.coreComp H)).symm ≪≫ (F.coreComp (G.comp H)).symm