For full description see:

http://www.daisy.org/epub/issues/use-relax-ng-text-pattern-schema-title-identifi
er-and-others

When we're drafting OPF language this time around, it seems likely we should 
disallow empty content in the four required "dc:" metadata elements (e.g. 
dc:title, dc:identifier); we already have required content verbiage for 
dc:language.