code: def set_representative_cases(self, ns, gs, np):
          
          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
docs: >>> 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: