; Write largest-in-lists, which takes a list of lists of numbers
; and finds the largest number.  Returns  #f if there are no numbers
; in any of the lists. Use letrec or named let. If you have time,
; write multiple versions with different approaches.


; Tests:
(largest-in-lists '())
(largest-in-lists '((1 3 5) () (4) (2 6 1) (3)))
(largest-in-lists '(() ()))