(define make-constant
  (lambda (x) x))

(define constant? number?)

(define make-expr
  (lambda (left-operand operator right-operand)
    (list left-operand operator right-operand)))

(define operator cadr)

(define left-operand car)

(define right-operand caddr)

(define evaluate
  (lambda (expr)
    (cond ((constant? expr) expr)
          (else ((look-up-value (operator expr))
                 (evaluate (left-operand expr))
                 (evaluate (right-operand expr)))))))

(define look-up-value
  (lambda (name)
    (cond ((equal? name 'plus) +)
          ((equal? name 'times) *)
          ((equal? name 'minus) -)
          ((equal? name 'divide) /)
          ((equal? name 'reversedminus) (lambda (x y)
                                          (- y x)))
          (else (error "Unrecognized name" name)))))

(define operators-onto
  (lambda (expr lst)
    (if (constant? expr)
        lst
        (cons (operator expr)
              (operators-onto (left-operand expr)
                              (operators-onto (right-operand expr)
                                              lst))))))

(define operators
  (lambda (expr)
    (operators-onto expr '())))