Using XPath 3.1 in Assertions
If the attribute saxon:extensions="any-xpath-version"
is present on the xs:schema
element of a schema document, then XPath expressions used within assertions and conditional type assignments
within that schema document are allowed to use XPath 3.1 syntax. By default, only XPath 2.0 is allowed.
This feature is new in Saxon 9.8.0.5.
The saxon:extensions
attribute is a whitespace-separated list of keywords.