[racket] writing a define-world macro

From: Shriram Krishnamurthi (sk at cs.brown.edu)
Date: Mon Jul 26 08:19:08 EDT 2010

If you're satisfied with that, then simple updaters will do the trick,
and you'll be fine.  But in practice you tend to have complex nested
structures, and this form of updaters turns out to be rather less


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