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An ID is conceptually required on start-of-range indexterms, is it practical to make the xml:id required in the RELAX NG for this pattern?
Can't do this in in 5.1 either. FU for 6.0 per http://lists.oasis-open.org/archives/docbook-tc/200912/msg00024.html
Fixed in 5.1
Oops. Nope. It's not b/w compatible. Leave for 6.0.