(define reduce-once-appl
  (lambda (exp succeed fail)
    (variant-case exp
      (lit (datum) (fail))
      (varref (var) (fail))
      (lambda (formal body) (fail))
      (app 
       (rator rand)
       (reduce-once-appl 
        rator
        (lambda (reduced-rator)
          (succeed (make-app reduced-rator rand)))
        (lambda ()
          (reduce-once-appl 
           rand
           (lambda (reduced-rand)
             (succeed (make-app rator reduced-rand)))
           (lambda ()
             (if (beta-redex? exp)
                 (succeed (beta-reduce exp))
                 (fail)))))))
      (else (error "unknown AST type" exp)))))