function exportValue(&$submitValues, $assoc = false)
    {
        $value = $this->_findValue($submitValues);
        if (null === $value) {
            $value = $this->getValue();
        } elseif (is_array($this->_values) && ($value != $this->_values[0]) && ($value != $this->_values[1])) {
            $value = null;
        }
        return $this->_prepareValue($value, $assoc);
    }