code: def get_representative_cases(self):
          
          return (self.get_declined(decl_utils.Case.nominative, decl_utils.Number.singular),
                  self.get_declined(decl_utils.Case.genitive, decl_utils.Number.singular),
                  self.get_declined(decl_utils.Case.nominative, decl_utils.Number.plural))
docs: >>> armr = OldNorseNoun("armr", decl_utils.Gender.masculine)
          >>> armr.set_representative_cases("armr", "arms", "armar")
          >>> armr.get_representative_cases()
          ('armr', 'arms', 'armar')
  
          :return: nominative singular, genetive singular, nominative plural