function setXprop( $label, $value, $params=FALSE ) {
    if( empty( $label ) || empty( $value ))
      return;
    $xprop = array( 'value' => $value );
    $toolbox = new calendarComponent();
    $xprop['params'] = $toolbox->_setParams( $params );
    $this->xprop[$label] = $xprop;
  }