So I first try to write directly a "right" xsl file, and if it works,
it means my intuition about koha-indexdefs-to-zebra.xsl is good and
that it needs to be updated (and I don't know how to do that)
You have 3 files:
1. xsl/koha-indexdefs-to-zebra.xsl
2. marc_defs/unimarc/biblios/biblio-koha-indexdefs.xml
3. marc_defs/unimarc/biblios/
biblio-zebra-indexdefs.xsl
If you modify DOM indexing, you edit file 2, and then you generate file
3 from 2 using 1, using a XSLT processor, for example xsltproc:
xsltproc xsl/koha-indexdefs-to-zebra.xsl \\
marc_defs/unimarc/biblios/biblio-koha-indexdefs.xml \\
> marc_defs/unimarc/biblios/biblio-zebra-indexdefs.xsl