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S9 LIB (make-queue) ==> queue
(queue! queue object) ==> unspecific
(unqueue! queue) ==> object
(queue-empty? queue) ==> queue
(unqueue* queue) ==> list
MAKE-QUEUE returns an empty queue. A queue is a data structure
that delivers unqueued elements in the same order in which they
were queued (a first-in first-out structure). Both enqueue and
dequeue operations are O(1).
QUEUE! (destructively) inserts an OBJECT at the input end
of QUEUE.
QUEUE-EMPTY? returns #T if QUEUE is empty.
UNQUEUE! (destructively) removes an element from the output end
of QUEUE and returns it.
UNQUEUE* removes an element from a queue and returns both the
element and the queue in a list of the form
(element queue)
NOTE: although a queue may look like a list, it is actually
a (directed acylic) graph. Altering a queue with list operations
is therefore not recommended! Copying a queue with procedures
like TREE-COPY will turn it into a non-queue. Caveat utilitor!
(let ((q (make-queue)))
(for-each (lambda (x) (queue! q x))
'(a b c d e))
(unqueue* q)) ==> (a ((e) b c d e))
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