Implementing support for filterin/filterout in assemblies

[Peter Desjardins <peter.desjardins.us@gmail.com>]

Hi! I need to do some conditional processing of a book that I'm
assembling from an assembly structure. A specific audience needs a
version of the book with some modules removed.

I see in https://tdg.docbook.org/tdg/5.1/ch06.html that the filterout
element is intended to do what I need. But it looks like support for
filterout hasn't been implemented yet:

https://github.com/docbook/xslt10-stylesheets/blob/master/xsl/assembly/assemble.xsl#L742

I am about to fork the repository and try hacking together some form
of support. Before I do, has anyone else done this? If possible, I'd
like to help work on something that could be contributed to the
project.

Thanks!

Peter