Queues
******

   Queues are declared as an "abstract" class. They are currently
implemented in any of three ways.

`VQueue'
     implement fixed sized Queues via arrays.

`XPQueue'
     implement dynamically-sized Queues via XPlexes.

`SLQueue'
     implement dynamically-size Queues via linked lists.

   All possess the same capabilities; they differ only in constructors.
`VQueue' constructors require a fixed maximum capacity argument.
`XPQueue' constructors optionally take a chunk size argument.
`SLQueue' constructors take no argument.

   Assume the declaration of a base element `x'.

`Queue q; or Queue q(int capacity);'
     declares a queue.

`q.empty()'
     returns true if queue q is empty.

`q.full()'
     returns true if queue q is full. XPQueues and SLQueues are never
     full.

`q.length()'
     returns the current number of elements in the queue.

`q.enq(x)'
     enqueues x on queue q.

`x = q.deq()'
     dequeues and returns the front of queue

`q.front()'
     returns a reference to the front of queue.

`q.del_front()'
     dequeues, but does not return the front of queue

`q.clear()'
     removes all elements from the queue.