QUEUE-ADD (QUEUE OBJECT)
Adds an object to the end of the queue.
Removes all elements from the queue (and returns the empty queue).
Returns true if the queue is empty, otherwise nil.
QUEUE-PEEK (QUEUE &OPTIONAL (TYPE 'T))
Returns the first object in the queue that has the given type (and removes all objects from the queue before it). Returns NIL (and clears the entire queue) if there is no such object.
QUEUE-REMOVE (QUEUE &KEY ERRORP)
Returns the first (i.e. least recently added) element of the queue. If the queue is empty, it returns nil (when :ERRORP is nil) or signals an empty-queue-error (when :ERRORP is true).
QUEUE-ADD-AT-FRONT (QUEUE OBJECT)
A queue of the ids of all non-dirty objects that are currently in the cache memory. Whenever an object is retrieved (i.e. read), it's added to the queue. If an object-id is in this queue, it is not necessarily in the OBJECTS hash-table.