sicp-ex-5.7

  
  
 this is another test for expt-machine: 
 (set-register-contents! expt-machine 'b 3) 
 (set-register-contents! expt-machine 'n 4) 
 (start expt-machine) 
 'done 
 'done 
 'done 
 > (get-register-contents expt-machine 'r) 
 81