code: def tag(self, name, formatter=None):
          
          tag = Tag(name, formatter)
          for tag_data in self._tags:
              if tag_data.name == name:
                  tag = tag_data
                  break
          else:
              self._tags.append(tag)
          return tag
docs: Return instance of Tag.
  
          Args:
              name (str): The value for this tag.
              formatter (method, optional): A method that take a tag value and returns a
                  formatted tag.
  
          Returns:
              obj: An instance of Tag.