Exercise 5.24.  Implement cond as a new basic special form without 
reducing it to if. You will have to construct a loop that tests the 
predicates of successive cond clauses until you find one that is true, 
and then use ev-sequence to evaluate the actions of the clause.

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ev-cond
  (assign unev (op cond-clauses) (reg exp))
ev-cond-loop
  (test (op empty-cond-expr?) (reg unev))
  (branch (label ev-cond-empty))
  (save env)
  (assign exp (op first-cond-clause) (reg unev))
  (assign unev (op rest-of-cond-clauses) (reg unev))
  (save unev)                                       ; save the rest of the clauses
  (save exp)                                        ; save this entire clause
  (assign exp (op cond-clause-predicate) (reg exp)) ; extract the predicate
  (save continue)
  (assign continue (label ev-cond-after-predicate))
  (goto (label eval-dispatch))
ev-cond-empty
  (assign val (const #f))
  (goto (reg continue))
ev-cond-after-predicate
  (restore continue)
  (restore exp)
  (restore unev)
  (restore env)
  (test (op true?) (reg val))
  (branch (label ev-cond-pred-true))
  (goto (label ev-cond-loop))
ev-cond-pred-true
  (assign exp (op cond-clause-sequence) (reg exp))
  (goto (label eval-dispatch))