bibliographic use case: final version (?)

I've attached the final (?) revision of the bibliographic requirements, 
incorporating suggestions from the call (and adding the "round- 
tripping" bit). In particular, I changed the typing bit to say 
"resource descriptions must be typed (book, article, legal case, etc.) 
in a standardized-but-extensible way" to cover Rob's comment that we 
want to allow flexibility.

There's no either/or choice here (flexibility vs. control), since in 
RELAX NG we can just do:

	reference_types = "Article" | "Book" | reference_types.extended
	reference_types.extended = notAllowed

... providing for the extension point.

Of course, I'd want those types to be URIs; e.g.:

	http://purl.org/net/biblio#Book

[points to a comprehensive type schema I wrote for just this purpose; 
though I need to get the host to fix it so that it can correctly 
resolve the RDF schema fragment there]

Then if some developer had a type schema they wanted to use instead for 
some reason, they could just plug in whatever they wanted.

Anyway, a round-about way of explaining my word choice in the document 
;-)

Bruce