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

def add(self, fragment_info, file_info):
        """
        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
        """
        if self.is_cached(fragment_info):
            raise ValueError(u"Attempt to add text already cached")
        self.cache[fragment_info] = file_info