Saxon中的XQuery的查询语句示例，如何清除global variable??

            String sexp = "declare variable $groupName as xs:string external; " + "/n" +
                "for $t in //quartz/job/job-detail" + " /n" +
                "where $t/group = $groupName" + " /n" +
                "return $t/name";
           
            XQueryExpression exp = staticContext
                    .compileQuery(sexp);
            dynamicContext.clearParameters();
            dynamicContext.setParameter("groupName", groupName);