;; Jacob Mehr's preorder procedure
(define pre-order
  (lambda (tree)
    (define pre-order-onto
      (lambda (tree list)
        (if (constant? tree)
            (cons tree list)
            (cons (operator tree) 
                  (append (pre-order-onto 
                           (left-operand tree)
                           (append (pre-order-onto (right-operand tree) list))))))))
    (pre-order-onto tree '())))