Exercise 5.19.  Alyssa P. Hacker wants a breakpoint feature in the 
simulator to help her debug her machine designs. You have been hired to 
install this feature for her. She wants to be able to specify a place in 
the controller sequence where the simulator will stop and allow her to 
examine the state of the machine. You are to implement a procedure

(set-breakpoint <machine> <label> <n>)

that sets a breakpoint just before the nth instruction after the given 
label. For example,

(set-breakpoint gcd-machine 'test-b 4)

installs a breakpoint in gcd-machine just before the assignment to 
register a. When the simulator reaches the breakpoint it should print 
the label and the offset of the breakpoint and stop executing 
instructions. Alyssa can then use get-register-contents and 
set-register-contents! to manipulate the state of the simulated machine. 
She should then be able to continue execution by saying

(proceed-machine <machine>)

She should also be able to remove a specific breakpoint by means of

(cancel-breakpoint <machine> <label> <n>)

or to remove all breakpoints by means of

(cancel-all-breakpoints <machine>)

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skipped.