private function _const() {
    $node = new ConstantDeclarationStatementNode();
    $this->matchDocComment($node);
    $this->mustMatch(T_CONST, $node);
    $declarations = new CommaListNode();
    do {
      $declarations->addChild($this->constDeclaration());
    } while ($this->tryMatch(',', $declarations));
    $node->addChild($declarations, 'declarations');
    $this->endStatement($node);
    return $node;
  }