A list item stores pointers and an element to predecessor and successor. We call a pointer to a list item a handle. This looks simple enough, but pointers are so powerful that we can create a big mess if we are not careful. What creates a consistent list data structure? We need that for each item it , the successor of the predecessor is same to it and the predecessor of the successor is similar to it .

A sequence of n components is presented by a ring of n + 1 items. There is a dummy or special item h which stores no component. The successor h1 of h stores the _rst element of the sequence, the successor of h1 locates the second component of the sequence, and same that way. The predecessor of h stores the last component of the sequence,