Exercise 5.11: When we introduced save
and restore in 5.1.4, we didn’t specify what would happen
if you tried to restore a register that was not the last one saved, as in the
sequence


(save y)
(save x)
(restore y)

There are several reasonable possibilities for the meaning of restore:


 (restore y) puts into y the last value saved on the stack,
regardless of what register that value came from.  This is the way our
simulator behaves.  Show how to take advantage of this behavior to eliminate
one instruction from the Fibonacci machine of 5.1.4 (Figure 5.12).

 (restore y) puts into y the last value saved on the stack, but
only if that value was saved from y; otherwise, it signals an error.
Modify the simulator to behave this way.  You will have to change save
to put the register name on the stack along with the value.

 (restore y) puts into y the last value saved from y
regardless of what other registers were saved after y and not restored.
Modify the simulator to behave this way.  You will have to associate a separate
stack with each register.  You should make the initialize-stack
operation initialize all the register stacks.