Common abstract document class

[<Frederick.Hirsch@nokia.com>]

RE Core, issue #4 in my earlier comments on draft 11

Trevor

The reason for recommending a common abstract document base class,
for Document and DocumentHash, is to clarify commonality in schema, avoid maintenance errors going forward (same thing not defined in two places) and  allow potential new derived document types.

I think the cost in additional lines is worthwhile.

regards, Frederick

Frederick Hirsch
Nokia