function asString()
{
$attribs = array();
if ($this->term !== null) {
$attribs['term'] = $this->term;
}
if ($this->scheme !== null) {
$attribs['scheme'] = $this->scheme;
}
if ($this->label !== null) {
$attribs['label'] = $this->label;
}
$xs = new XMLStringer();
$xs->element('category', $attribs);
return $xs->asString();
}