private function condition(ParentNode $node, $property_name) {
    $this->mustMatch('(', $node, 'openParen');
    if ($this->currentType === T_YIELD) {
      $node->addChild($this->_yield(), $property_name);
    }
    else {
      $node->addChild($this->expr(), $property_name);
    }
    $this->mustMatch(')', $node, 'closeParen');
  }