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''Update functions'' in Scheme.

   Date: Thu, 19 May 88 10:35:23 EDT
   From: Mitchell Wand <wand%corwin.ccs.northeastern.edu@relay.cs.net>

   One ought not to say things like:

   "F(G(C)) := D ought to ensure that F(G(C)) = D afterwards."

   too blithely.  Consider the array assignment:

   A[A[1]] := 2

   in a two element array A, where initially A[1]=A[2]=1 .  This sort of thing had
   program verifiers confused for a good while in the early 70's.

   Mitchell Wand
   College of Computer Science
   Northeastern University
   360 Huntington Avenue #161CN
   Boston, MA 02115

   CSNet:  wand@corwin.ccs.northeastern.edu

Now consider the array assignment  Y(A[.]) := 2 .  :-)