protected function _toUpdate($mailbox, $type, $data)
    {
        if (!isset($this->_update[$mailbox])) {
            $this->_update[$mailbox] = array(
                'add' => array(),
                'slice' => array()
            );
        }

        $this->_update[$mailbox][$type] = ($type == 'slicemap')
            ? $data
            : array_merge($this->_update[$mailbox][$type], $data);
    }