Assume that we make a URN which names a document,
when I request information about that document/product, I
specify a version number, *or* I ask for the current version.
Functionally this would be:
<registrar://ora/nutshell books/some book> is an implicit
reference to the current version
<registrar://ora/nutshell books/some book:2.0> is an
explicit reference to something which does not change,
ie. version 2.0 of the book and
<registrar://ora/nutshell books/some book:0.0> is an
explicit reference to the current version
</rr>