code: def append_to_arg_count(self, data):
          
          assert data in '-0123456789'
          current = self._arg
  
          if data == '-':
              assert current is None or current == '-'
              result = data
          elif current is None:
              result = data
          else:
              result = "%s%s" % (current, data)
  
          self.input_processor.arg = result
docs: Add digit to the input argument.
  
          :param data: the typed digit as string