https://purengom.com/2009/07/27/%eb%af%b8%eb%94%94%ec%96%b4%eb%b2%95-%eb%85%bc%eb%9e%80-%ea%b7%b8%eb%a6%ac%ea%b3%a0-%ec%a7%80%ec%83%81%ed%8c%8c%ec%99%80-%ec%a2%85%ed%95%a9%ed%8e%b8%ec%84%b1-%ec%b1%84%eb%84%90%ec%97%90-%eb%8c%80/