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[plt-scheme] references to deleted objects

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Mon Nov 11 10:56:01 EST 2002		 
At Thu, 7 Nov 2002 11:50:50 -0800 (PST), Ron Stanonik wrote:
> 2) Add a "deleted" field to oval%, initialize it to #t, have the
>    delete method set deleted to #f, and have the other oval% methods
>    check deleted before trying to operate on the missing graphic object.

This is what most MrEd methods do. It's not great, but I don't have a
better idea myself.

Matthew
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