TExpression class file

• version: $Id: TExpression.php 1398 2006-09-08 19:31:03Z xue $
• license: http://www.pradosoft.com/license/
• copyright: Copyright © 2005 PradoSoft
• link: http://www.pradosoft.com/
• author: Qiang Xue <qiang.xue@gmail.com>