[Om] Content-dictionary notations draft

[David Carlisle]

> actually I missed the fact that it's to be aside... do you care to  
> elaborate why ? Or better... why only ??
> I'm fine to have it in such an annex as an STS file!

for OM I think it should be in a separate file cf the sts files, for the
same reason, to maintain the distance between the semantic/content
nature of the CD and any possible rendering. However in the parallel
discussion about a CD-like notation for mathml, it makes sense for it to
be in the same file, as MathML has an immediate relationship with the
presentation form, and historically (unlike openmath) every content
mathml expression has a default presentation.  If we want (at some point
in the future) to say that the MathML CD and syntax are alternative
concrete syntaxes for the abstract OpenMath and OpenMath CD models, this
may mean that the abstract CD model needs to allow presentation
specification, even if the XML OpenMath CD format does not...

David