Prevent schematron-related SXWN9000 Saxon error

[sideshowbarker]

Fixes #240
See #240 (comment)

[rdeltour]

I was originally reluctant to suggest directly touching the XSLT, since I didn't know where it came from or whether it was expected to be in sync with whatever upstream version… but looking closer at the code I concur with @georgebina that the proposed change s/b safe.
And I agree it’s better than #241 for the reasons stated by @ndw 😊

So LGTM!

[georgebina]

We should apply the same fix to the ISO Schematron stylesheet - I know we do not fully support ISO Schematron, but we should keep the two stylesheets in sync.

[sideshowbarker]

We should apply the same fix to the ISO Schematron stylesheet - I know we do not fully support ISO Schematron, but we should keep the two stylesheets in sync.

OK, yep, done — and (force-)pushed to the branch in c1d3486

If we get agreement from @ndw on this, I’ll go ahead and merge it master and do a new release

[ndw]

If it passes tests and gets the blessing of my esteemed colleagues, 👍