(define (rl) (load "subst.ss"))

(define subst-leftmost
  (lambda (new old ls pred)
    (letrec ([do-cdr (lambda (ls)
		       (let ([result-cdr (local (cdr ls))])
			 (cons (car result-cdr) (cons (car ls) (cdr result-cdr)))))]
	     [local (lambda (ls)
		      (cond [(null? ls) '(#f)]
			    [(or (symbol? (car ls))
				 (number? (car ls)))
			     (if (pred old (car ls))
				 (cons #t (cons new (cdr ls)))
				 (do-cdr ls))]
			    [else (let ([result (local (car ls))])
				    (if (car result)
					(cons #t (cons (cdr result) (cdr ls)))
					(do-cdr ls)))]))])
      (cdr (local ls)))))