code: def desc(self, description):
          '''Describe a parser, when it failed, print out the description text.'''
          return self | Parser(lambda _, index: Value.failure(index, description)f desc(self, description):
          '''Describe a parser, when it failed, print out the description text.'''
          return self | Parser(lambda _, index: Value.failure(index, description))
docs: Describe a parser, when it failed, print out the description text.