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Re: ''Update functions'' in Scheme.

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