* Use the LibreOffice Relax-NG files since they include a hack to
support 1.2.
* rng source:
https://raw.githubusercontent.com/freedesktop/libreoffice-core/master/
schema/libreoffice/OpenDocument-schema-v1.3%2Blibreoffice.rng
* translation to rnc with trang:
trang -I rng -O rnc OpenDocument-schema-v1.3+libreoffice.rng \
OpenDocument-schema-v1.3+libreoffice.rnc