Extra-Grammatical Constraints

Exclusions. (High on my list of features for a future version of RELAX NG.)

A version attribute on the root element.

Enforcing implicit constraints. (In a segmented list, the number of segments in each list item has to be the same as the number of titles specified.)

Enforcing referential integrity constraints. (A cross-reference on a footnoteref must point to a footnote.)