e tuples (flags, value) as values if
            C{withIdentifier} is C{False}, or (flags, cas identifier, value) if
            C{True}.  If the server indicates there is no value associated with
            C{key}, the returned values will be L{None} and the returned flags
            will be C{0}.
        @rtype: L{Deferred}

        @since: 9.0
        Tr„