[16] XQuery 1.0 and XPath 2.0 Functions and Operators (Second Edition) ( ( 版)) <http://www.w3.org/TR/2010/REC-xpath-functions-20101214/#func-id>
[17] XQuery 1.0 and XPath 2.0 Functions and Operators (Second Edition) ( ( 版)) <http://www.w3.org/TR/2010/REC-xpath-functions-20101214/#func-element-with-id>