CCFQueueSourceQueue containing elements of type 'a
Same as take_front, but fails on empty queues.
take_front_l n q takes at most n elements from the front of q, and returns them wrapped in a list.
take_back_l n q removes and returns the last n elements of q. The elements are in the order of the queue, that is, the head of the returned list is the first element to appear via take_front. take_back_l 2 (of_list [1;2;3;4]) = of_list [1;2], [3;4].
Append two queues. Elements from the second one come after elements of the first one. Linear in the size of the second queue.