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At present, area requires an xml:id and my have a link
attribute. Areaset also requires an xml:id and may not
have a link attribute.

I'd like to make the following changes:

1. Make the xml:id optional on area inside an areaset.

2. Add link attributes to areaset

3. Change the semantics of area so that it inherits
linking attributes from the areaset that contains it
(if it's contained in an areaset).