Saxon

It's not clear how to invoke a 4.0 stylesheet using fn:transform.

Generally, we've been changing the code to do what the documentation says: to run XSLT 4.0, you have to request it in the invocation API; it's not enough to request it in the xsl:stylesheet/@version.

This is slightly at odds with the fn:transform specification, which suggests that if no specific version is requested in the fn:transfom options, then xsl:stylesheet/@version is used. However it think it makes sense to require a very explicit request at this stage of 4.0 development, given that lots of change in the spec can still be expected.

So I'm changing fn:transform to align with the Java/C# API. To use 4.0 capabilities in the called stylesheet, you must (a) have 4.0 enabled in the calling stylesheet, and (b) request `xslt-version="4.0" in the fn:transform options.