A side effect is an action of a program other than returning a value:

• Changing a global variable: (incf *count*)

• Printing or other I/O

• Modifying structure: (setf (car arg) 'foo)

An otherwise correct transformation may not be correct if the code to be transformed has side-effects.

( (- (+ ?x ?y) (+ ?x ?z))    (- ?y ?z) )

(- (+ (progn (print 'foo) 3) y)
   (+ (progn (print 'foo) 3) z))

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