public function __isset($var)
    {
        // Look for access of the form {ns:var}. We don't use
        // _children() here because we can break out of the loop
        // immediately once we find something.
        if (strpos($var, ':') !== false) {
            list($ns, $elt) = explode(':', $var, 2);
            foreach ($this->_element->childNodes as $child) {
                if ($child->localName == $elt && $child->prefix == $ns) {
                    return true;
                }
            }
        } else {
            foreach ($this->_element->childNodes as $child) {
                if ($child->localName == $var) {
                    return true;
                }
            }
        }
    }