fix break-at to allow empty breakpoint list as input

list.rkt

@@ -94,7 +94,7 @@
 (define increasing-positive-list? (and/c list? increasing-positive?))
 
 (define+provide/contract (break-at xs bps)
-  (list? (and/c coerce/list? increasing-positive-list?) . -> . (listof list?))
+  (list? (and/c coerce/list? (or/c empty? increasing-positive-list?)) . -> . (listof list?))
   (when (ormap (λ(bp) (>= bp (length xs))) bps)
     (error 'break-at (format "breakpoint in ~v is greater than or equal to input list length = ~a" bps (length xs))))
   ;; easier to do back to front, because then the list index for each item won't change during the recursion

tests.rkt

@@ -162,6 +162,8 @@
 (check-equal? (sublist (range 5) 0 1) '(0))
 (check-equal? (sublist (range 5) 0 5) '(0 1 2 3 4))
 
+(check-equal? (break-at '(5 6 7 8) '()) '((5 6 7 8)))
 (check-equal? (break-at '(5 6 7 8) '(1 2 3)) '((5) (6) (7) (8)))
 (check-equal? (break-at '(5 6 7 8) '(1 3)) '((5) (6 7) (8)))
 (check-equal? (break-at '(5 6 7 8) '(1)) (break-at '(5 6 7 8) 1))
+