[osis-core] shouldn't osisRef be required for <reference>

[Chris Little]

On Wed, 28 May 2003, Todd Tillinghast wrote:

> I just noticed that osisRef is an optional attribute of <reference>.
> Shouldn't osisRef be a required attribute of <reference>?

Not making osisRef a required attribute of <reference> allows marking 
references to works for which no canonical reference system exists and of 
which no OSIS document has yet been encoded.

I'm not saying it's necessarily good to mark references like this, but 
this is the benefit of osisRef being optional.

--Chris