<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def expire_cache(fragment_name, *args):
 """ Expire a cache item. @param url: The url object @param product_names: A list of product names @param start: The date from which the reporting should start. @param stop: The date at which the reporting should stop. """

    cache_key = make_template_fragment_key(fragment_name, args)
    cache.delete(cache_key)