<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cas(self, key, value, cas, expire=0, noreply=False):
 """ The memcached "cas" command. Args: key: str, see class docs for details. value: str, see class docs for details. cas: int or str that only contains the characters '0'-'9'. expire: optional int, number of seconds until the item is expired from the cache, or zero for no expiry (the default). noreply: optional bool, False to wait for the reply (the default). Returns: If noreply is True, always returns True. Otherwise returns None if the key didn't exist, False if it existed but had a different cas value and True if it existed and was changed. """

        return self._store_cmd(b'cas', {key: value}, expire, noreply, cas)[key]