def set_representative_cases(self, ns, gs, np):
        """
        >>> armr = OldNorseNoun("armr", decl_utils.Gender.masculine)
        >>> armr.set_representative_cases("armr", "arms", "armar")

        >>> armr.declension
        [['armr', '', '', 'arms'], ['armar', '', '', '']]

        :param ns:
        :param gs:
        :param np:
        :return:
        """
        self.set_void_declension(decl_utils.Number, decl_utils.Case)
        self.declension[decl_utils.Number.singular.value-1][decl_utils.Case.nominative.value-1] = ns
        self.declension[decl_utils.Number.singular.value-1][decl_utils.Case.genitive.value-1] = gs
        self.declension[decl_utils.Number.plural.value-1][decl_utils.Case.nominative.value-1] = np