A breakpoint feature

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 n th 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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