Stacks
******

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

`VStack'
     implement fixed sized stacks via arrays.

`XPStack'
     implement dynamically-sized stacks via XPlexes.

`SLStack'
     implement dynamically-size stacks via linked lists.

   All possess the same capabilities. They differ only in constructors.
VStack constructors require a fixed maximum capacity argument.  XPStack
constructors optionally take a chunk size argument.  SLStack
constructors take no argument.

   Assume the declaration of a base element `x'.

`Stack s;  or Stack s(int capacity)'
     declares a Stack.

`s.empty()'
     returns true if stack s is empty.

`s.full()'
     returns true if stack s is full. XPStacks and SLStacks never
     become full.

`s.length()'
     returns the current number of elements in the stack.

`s.push(x)'
     pushes x on stack s.

`x = s.pop()'
     pops and returns the top of stack

`s.top()'
     returns a reference to the top of stack.

`s.del_top()'
     pops, but does not return the top of stack. When large items are
     held on the stack it is often a good idea to use `top()' to
     inspect and use the top of stack, followed by a `del_top()'

`s.clear()'
     removes all elements from the stack.