N-nary Tree
An n-nary tree T is a finite set of nodes with the following properties:
1. Either the set id empty or
2. The set consists of a root, R, and exactly N distinct N-ary trees. That is, the remaining nodes are partitioned into N>= 0 sunbsets T0, T1, .. TN-1, each of which is an N-ary tree such that T = {R, T0, T1, .. TN-1}.