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 
  
    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⟩)