Sometimes context information can be sent top-down to allow transformations to be made safely.

• Logical use of result

• Result is busy outside

( (if (not (not ?p)) ?x)  (if ?p ?x) )

( (progn (rplaca ?x ?y)   (progn
         ?u . ?z)           (setf (car ?x) ?y)
                            ?u . ?z) )

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