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(slib.info)Queues

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Queues
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  `(require 'queue)'

  A "queue" is a list where elements can be added to both the front and
rear, and removed from the front (i.e., they are what are often called
"dequeues").  A queue may also be used like a stack.

 - Function: make-queue
     Returns a new, empty queue.

 - Function: queue? obj
     Returns `#t' if OBJ is a queue.

 - Function: queue-empty? q
     Returns `#t' if the queue Q is empty.

 - Procedure: queue-push! q datum
     Adds DATUM to the front of queue Q.

 - Procedure: enquque! q datum
     Adds DATUM to the rear of queue Q.

  All of the following functions raise an error if the queue Q is empty.

 - Function: queue-front q
     Returns the datum at the front of the queue Q.

 - Function: queue-rear q
     Returns the datum at the rear of the queue Q.

 - Prcoedure: queue-pop! q
 - Procedure: dequeue! q
     Both of these procedures remove and return the datum at the front
     of the queue.  `queue-pop!' is used to suggest that the queue is
     being used like a stack.