def expire(self, time=None):
        """Remove expired items from the cache."""
        if time is None:
            time = self.__timer()
        root = self.__root
        curr = root.next
        links = self.__links
        cache_delitem = Cache.__delitem__
        while curr is not root and curr.expire < time:
            cache_delitem(self, curr.key)
            del links[curr.key]
            next = curr.next
            curr.unlink()
            curr = next
Remove expired items from the cache.