Linked Lists
************

   SLLists provide pseudo-generic singly linked lists.  DLLists provide
doubly linked lists.  The lists are designed for the simple maintenance
of elements in a linked structure, and do not provide the more extensive
operations (or node-sharing) of class `List'. They behave similarly to
the `slist' and similar classes described by Stroustrup.

   All list nodes are created dynamically. Assignment is performed via
copying.

   Class `DLList' supports all `SLList' operations, plus additional
operations described below.

   For purposes of illustration, assume the specification of class
`intSLList'. In addition to the operations listed here, SLLists support
traversal via Pixes. See Pix

`intSLList a;'
     Declares a to be an empty list.

`intSLList b = a;'
     Sets b to an element-by-element copy of a.

`a.empty()'
     returns true if a contains no elements

`a.length();'
     returns the number of elements in a.

`a.prepend(x);'
     places x at the front of the list.

`a.append(x);'
     places x at the end of the list.

`a.join(b)'
     places all nodes from b to the end of a, simultaneously destroying
     b.

`x = a.front()'
     returns a reference to the item stored at the head of the list, or
     triggers an error if the list is empty.

`a.rear()'
     returns a reference to the rear of the list, or triggers an error
     if the list is empty.

`x = a.remove_front()'
     deletes and returns the item stored at the head of the list.

`a.del_front()'
     deletes the first element, without returning it.

`a.clear()'
     deletes all items from the list.

`a.ins_after(Pix i, item);'
     inserts item after position i. If i is null, insertion is at the
     front.

`a.del_after(Pix i);'
     deletes the element following i. If i is 0, the first item is
     deleted.

Doubly linked lists
===================

   Class `DLList' supports the following additional operations, as well
as backward traversal via Pixes.

`x = a.remove_rear();'
     deletes and returns the item stored at the rear of the list.

`a.del_rear();'
     deletes the last element, without returning it.

`a.ins_before(Pix i, x)'
     inserts x before the i.

`a.del(Pix& iint dir = 1)'
     deletes the item at the current position, then advances forward if
     dir is positive, else backward.