## ABSTRACT

As we have seen above, the tree is a multilevel object, whose deﬁnition is based on mathematical induction. Indeed, assuming that the node at the head of the tree is given, then the entire tree is constructed by issuing edges (branches) from the head, and placing a smaller tree (a subtree) at the other end of each branch. Thanks to the induction hypothesis, one may assume that the deﬁnition of the subtrees is available. As a result, the entire tree is also well deﬁned.