http://www.cnblogs.com/ywqu/archive/2009/12/22/1629426.html
http://pic.dhe.ibm.com/infocenter/rhaphlp/v7r5/index.jsp
http://www-01.ibm.com/support/docview.wss?uid=swg21325950
return
timeout
message/event
execution occurrence:控制焦点是顺序图中表示时间段的符号,在这个时间段内对象将执行相应的操作。用小矩形表示.