Reduce, reduce-left and reduce-right

Students are confused about Exercise 6 in Lists lab, so I think I should give more explanation on this. First, some defs:

(define combine
  (lambda (s1 s2)
    (string-append s1 " and " s2 " and " s1)))

(define letters
  (list  "c" "d" "e"))

And if you use reduce-right:

> (reduce-right combine letters)
"c and d and e and d and c"

Recall that reduce-right starts processing from the right. Let’s do a run through:

> (reduce-right combine (list  "c" "d" "e"))
; as "e" is the far right element in the list, "d" is the second, reduce executes   (combine "e" "d") first. Then the procedure is actually this: 
> (reduce-right combine (list  "c" "d and e and d"))
; and now reduce execute (combine "c" "d and e and d") and yeah we run out of the list. Therefore, we have our result now: 
> "c and d and e and d and c"

; Similarly, reduce-left picks "c" and "d" as the first two elements. 
> (reduce-left combine (list  "c" "d" "e"))
> (reduce-left combine (list  "c and d and c" "e"))
; Notice the "c and d and c" is now the str1 passed into combine procedure
> "c and d and c and e and c and d and c"