2857	 Referencing Lists by an Edge	 An edge reference into a list structure is a pair of pointers to adjacent nodes. Such a reference often requires little additional space but its use can yield efficient algorithms. For instance a circular link between the ends of a list is redundant if the list is always referenced by that edge and list traversal is easier when that link is null. Edge references also allow threading of nonrecursive lists can replace some header cells and enhance the famous exclusive-or-trick to double-link lists list processing circular doubly linked overlapping sublist header cell pointer cursor
