Double ended Queues
*******************

   Deques are declared as an "abstract" class. They are currently
implemented in two ways.

`XPDeque'
     implement dynamically-sized Deques via XPlexes.

`DLDeque'
     implement dynamically-size Deques via linked lists.

   All possess the same capabilities. They differ only in constructors.
XPDeque constructors optionally take a chunk size argument.  DLDeque
constructors take no argument.

   Double-ended queues support both stack-like and queue-like
capabilities:

   Assume the declaration of a base element `x'.

`Deque d; or Deque d(int initial_capacity)'
     declares a deque.

`d.empty()'
     returns true if deque d is empty.

`d.full()'
     returns true if deque d is full.  Always returns false in current
     implementations.

`d.length()'
     returns the current number of elements in the deque.

`d.enq(x)'
     inserts x at the rear of deque d.

`d.push(x)'
     inserts x at the front of deque d.

`x = d.deq()'
     dequeues and returns the front of deque

`d.front()'
     returns a reference to the front of deque.

`d.rear()'
     returns a reference to the rear of the deque.

`d.del_front()'
     deletes, but does not return the front of deque

`d.del_rear()'
     deletes, but does not return the rear of the deque.

`d.clear()'
     removes all elements from the deque.