protected function parseIteratorExp ($exp, & $idxVar, & $itVar)
  {
    if (!preg_match ('/^(?:(\w+):)?(\w+)$/', $exp, $m))
      throw new ComponentException($this,
        "Invalid value for attribute <kbd>as</kbd>.<p>Expected syntax: <kbd>'var'</kbd> or <kbd>'index:var'</kbd>");
    list (, $idxVar, $itVar) = $m;
  }