Using XPath 3.1 in Assertions
From Saxon 9.9 the XPath version used can be configured using the configuration
If the attribute
saxon:extensions="any-xpath-version" is present on the
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 was new in Saxon 184.108.40.206.
saxon:extensions attribute is a whitespace-separated list of keywords.