Evolution of schema language

It's not clear to me (did I just forget?) whether Structural Requirement 6
("evolution of schemata") includes evolution of the schema language.  It
would be better if that requirement were clearer about what is and is not
intended.  Further, I think evolution of the schema language should be
supported.

The current Structures draft continues to say that the "model" of the
schema schema is "closed", meaning that the schema language is not
evolvable.  I think this is bad.

Mike Spreitzer <spreitze@parc.xerox.com>
http://parcweb.parc/spreitze/ (Xerox internal)
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