(module mp1-soln (lib "eopl.ss" "eopl")

  (provide subst)
  
  ;;;;;;;; Solution ;;;;;;;;;;;;;;
  
  ;; data definitions:
  ;;  slist ::= () OR (cons sexp slist)     -- ie, (list-of sexp).
  ;;  sexp    = symbol OR slist
  
  ;; subst : slist * symbol * slist -> slist
  ;; usage : (subst new old slist)
  ;; produces : a copy of slist with all occurrences of old
  ;; replaced by new.
  
  (define subst
    (lambda (new old slist)
      (cond
        [(null? slist)                  ; is the list empty?
         '()]                           ; yes: nothing to substitute
        [(pair? slist)                  ; no: work on the pieces
         (cons
           (subst-in-sexp new old (car slist))
           (subst new old (cdr slist)))])))

  ;; subst-in-s-exp : sym * sym * s-exp -> s-exp
  ;; usage: (subst-in-sexp new old sexp)
  ;; produces: a copy of sexp with all occurrences of old replaced by new

  (define subst-in-sexp
    (lambda (new old sexp)
      (if (symbol? sexp) 
        (if (eqv? sexp old) new sexp)
        (subst new old sexp))))

  )