Produce lists in increasing number of items from input list.

lists.rkt

(define (defsep los)
  (if (empty? (rest los))
      ""
      ", "))

(define (concat los)
  (cond [(empty? los) ""]
        [else
         (string-append (first los)
                        (defsep los)
                        (concat (rest los)))]))

(define (breadcrumb lol)
  (cond [(empty? lol) '()]
        [else
         (cons (concat lol)
               (breadcrumb (reverse (rest (reverse lol)))))]))

(breadcrumb '())
(breadcrumb '("one"))
(breadcrumb '("one" "two"))
(breadcrumb '("one" "two" "three"))
(reverse (breadcrumb '("one" "two" "three" "four")))

;; '()
;; (list "one")
;; (list "one, two" "one")
;; (list "one, two, three" "one, two" "one")
;; (list "one" "one, two" "one, two, three" "one, two, three, four")