Adding new elements into DocBook namespace

the most convenient way, but might cause element name clashes in future

# default namespace is DocBook
default namespace = "http://docbook.org/ns/docbook"

# import DocBook schema
include "docbook.rnc"

# two new elements are directly combined into pattern
a.inlines = element register { text }
            | element instruction { text }

# add new inlines to general inlines
db.general.inlines |= a.inlines