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.

————————————————————————————————————————————————————————————————————————

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

3/2

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

1/6

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

(1 (2 (3 ())))

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

(((() 1) 2) 3)

Associativity, (a op b) op c = a op (b op c).