Exercise 2.38: The accumulate procedure
is also known as fold-right, because it combines the first element of
the sequence with the result of combining all the elements to the right.  There
is also a fold-left, which is similar to fold-right, except that
it combines elements working in the opposite direction:


(define (fold-left op initial sequence)
  (define (iter result rest)
    (if (null? rest)
        result
        (iter (op result (car rest))
              (cdr rest))))
  (iter initial sequence))

What are the values of


(fold-right / 1 (list 1 2 3))
(fold-left  / 1 (list 1 2 3))
(fold-right list nil (list 1 2 3))
(fold-left  list nil (list 1 2 3))

Give a property that op should satisfy to guarantee that
fold-right and fold-left will produce the same values for any
sequence.