    # File lib/memcache.rb, line 175
      def cas(key, value, opts)
        raise 'opts must be hash' unless opts.instance_of?(Hash)
        key = key.to_s
        backup.cas(key, value, opts) if backup

        expiry = opts[:expiry] || default_expiry
        flags  = opts[:flags]  || 0
        data   = marshal(value, opts)
        server(key).cas(key, data, opts[:cas], expiry, flags) && value
      end