Consider a recursive data structure defined in a sequential language, such as C++, see fig. 2.1. The structure stores a value of type Data , and has pointers to two other structures of the same type, giving the definition a recursive nature. A null pointer would indicate that there was no further sub-structures in that direction.
This particular definition would define a binary tree structure, providing that the sub-structures are distinct from each other, and distinct from the parent structure. Now consider a way to generalise this structure, by allowing the pointers to point to different types of structure.
