List-structures that share list records as well as sharing sublists are frequently needed to fit the demands of a problem. These structures are more complicated than those presented thus far and may require large amounts of storage for their implementation. A natural idea is to avoid the duplication of identical sublists or records. At times it is convenient to use additional fields linking records or sublists so that the resultant structure contains records with more than one pointer to them, and the structure may even contain loops. That is, by following pointers from a record we can arrive back at the record. You will see examples of this in the case study that follows. Whenever these possibilities occur, the structures created are still lists but are no longer list-structures. These more general lists provide extensive flexibility but lead to difficult storage management problems, which are considered later in the text.