Building modules

Including not hard
include "mygrammar.rnc"
Include with override, redefining patterns
include "library.rnc" { start = element-book, author=element author{text} }
Referencing from within patterns:
book=element book {external "author.rnc"+, external "character.rnc"*}
Include, adding an extra option to a pattern
include "library.rnc" { start |= element-book }