mutable object model

What would people suggest I use for a mutable object model in Prolog?  Some specifics:

An open model is preferred, like is implicit in XML and JSON specs.
Simple XPath type queries are desired.  Select speed is paramount.

Data is public.  There are no methods as such.

Usually member variables would be treated logically: Initially unbound, then later bound.

At times selected variables will be reset to unbound.
Variables can share an object.

Usually objects are made by instantiating classes.

Standard serialization such as XML or JSON is desirable to communicate with other systems.

I don't expect everything above to be ready-provided.  I'm looking for whatever might 
be an efficient and simple foundation and I'll code the rest.

Alan Baljeu
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