I thought of 2 ways to solve this:
1) Have a list of safe operators that drop can be applied to and test against this within apply-generic.
2) Call drop within each package. We choose in the paackage implementation which procedures results can be dropped.

I prefer the latter because it feels wrong coupling the package implementations to apply-generic so tightly.
The authors specifically mention mpdifying apply-generic as the solution so I wrote:


(define (apply-generic op . args)
...
...
(define (reduce-type result)
(if (memq op '(add sub mul div))
(drop result)
result))
...
...
(if proc
(reduce-type (apply proc (map contents args)))
...
...


Anyway thanks for posting your solutions, it’s nice to check my solutions against someone else’s.