Exercise 5.6.  Ben Bitdiddle observes that the Fibonacci machine's 
controller sequence has an extra save and an extra restore, which can be 
removed to make a faster machine. Where are these instructions?

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Near afterfib-n-1:

 afterfib-n-1                         ; upon return, val contains Fib(n - 1)
   (restore n)
   (restore continue)
   ;; set up to compute Fib(n - 2)
   (assign n (op -) (reg n) (const 2))
   (save continue)
   (assign continue (label afterfib-n-2))

The restore restores a value which is then saved, but never used, which 
is useless.