Use of ontologies etc in the SNAP model
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In the UML model we introduce instance-s of Ontology.

These are assumed to refer to real ontologies, containing Term-s.

These instances are to be used by types in the model. Some of these have attributes 
whose type is "anyURI" and in their documentation indicate which Ontology instance
they should be obtained from.

This needs more discussion.
We may want to introduce a stereotype on an attribute which indicates that
the attribute takes its value in an externally maintained ontology.
This should come with a feature on the same attribute to indicate which ontology
is used. I have done so by creating a "Tag Definition", "ontologyURI" on the <<ontologyterm>>
sterotype. So when assigning that stereotype to an attribute, one can give taht
tag a value corresponding to the URL of the ontology within which it takes a value.

The type of the attribute must be anyURI or string. The latter should refer to the  
name of the term which we assume to be unique in the ontology. The anyURI would 
indicate the globally unique URI of the term.