def METHOD_NAME(self):
    r"""
    Return ``None``.
    Indeed, the category of bimodules defines no additional
    structure: a left and right module morphism between two
    bimodules is a bimodule morphism.
    .. SEEALSO:: :meth:`Category.additional_structure`
    .. TODO:: Should this category be a :class:`CategoryWithAxiom`?
    EXAMPLES::
        sage: Bimodules(QQ, ZZ).additional_structure()
    """
    return None