The value of the test attribute of the xs: assertion element facet is an XPath 2.0 expression or XPath 2.0 subset as defined by the XML Schema 1.1 specification that evaluates to either true or false.
如xml Schema 1.1规范定义的那样，xs： assertion元素facet的test属性的值是一个XPath 2.0表达式或XPath 2.0子集，其值为true或false。
The optional parameter can be either an element kind test or a schema-element kind test.
其参数可以是元素类别测试或者模式元素类别测试。
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