'Change the content of C{key} only if the C{cas} value matches the
current one associated with the key. Use this to store a value which
hasn\'t been modified since last time you fetched it.
@param key: The key to set.
@type key: L{bytes}
@param val: The value associated with the key.
@type val: L{bytes}
@param cas: Unique 64-bit value returned by previous call of C{get}.
@type cas: L{bytes}
@param flags: The flags to store with the key.
@type flags: L{int}
@param expireTime: If different from 0, the relative time in seconds
when the key will be deleted from the store.
@type expireTime: L{int}
@return: A deferred that will fire with C{True} if the operation has
succeeded, C{False} otherwise.
@rtype: L{Deferred}'

def checkAndSet(self, key, val, cas, flags=0, expireTime=0):

    return self._set('cas', key, val, flags, expireTime, cas)
