private function parameterList(ParentNode $parent) {
    $node = new CommaListNode();
    $this->mustMatch('(', $parent, 'openParen');
    $parent->addChild($node, 'parameters');
    if ($this->tryMatch(')', $parent, 'closeParen', TRUE)) {
      return;
    }
    do {
      $node->addChild($this->parameter());
    } while ($this->tryMatch(',', $node));
    $this->mustMatch(')', $parent, 'closeParen', TRUE);
  }