XPath运算符XPath运算符XPath运算符表达式可以返回节点集、字符串、逻辑值、数字。运算符描述实例含义|计算两个节点集//student/name|//student/age选取student元素的所有      name和age元素+加法/classroom/student[age=19+1]选取classroom元素的所有student元素，且