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);