We recently found a really important reason to subclass Model instead 
of Object for your models.  If you're going to use dependencies AND 
you want to save your system as a Project (.pr) file, then 
subclassing Object will break all your dependencies.  The global 
system dependencies are NOT saved with the Project.  But if you 
subclass Model, all your dependencies are stored within objects, so 
the saving to Projects works just fine.

Mark

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Mark Guzdial : Georgia Tech : College of Computing : Atlanta, GA 30332-0280
Associate Professor - Learning Sciences & Technologies.
Collaborative Software Lab - http://coweb.cc.gatech.edu/csl/
(404) 894-5618 : Fax (404) 894-0673 : guzdial at cc.gatech.edu
http://www.cc.gatech.edu/gvu/people/Faculty/Mark.Guzdial.html