code: def add(self, fragment_info, file_info):
          
          if self.is_cached(fragment_info):
              raise ValueError(u"Attempt to add text already cached")
          self.cache[fragment_info] = file_info
docs: Add the given ``(key, value)`` pair to the cache.
  
          :param fragment_info: the text key
          :type  fragment_info: tuple of str ``(language, text)``
          :param file_info: the path value
          :type  file_info: tuple ``(handler, path)``
          :raises: ValueError if the key is already present in the cache