xpath-version attribute

Hi all,

 

In between other work I'm trying to write something that might once become
the XProc 3 book. Coming from that I will sometimes fire questions about
things that are unclear to me in the specs. Here is the first:

 

You can set an xpath version in @xpath-version. Default is "3.1". What
exactly are processors supposed to do when they encounter a request for some
other Xpath version? 

I assume the only version an XProc 3 processor is required to support is
3.1. Supporting another (any other) version is implementation defined and
therefore not mandatory.

 

Am I correct?

 

Erik Siegel