By an unordered tree, 𝒯, we shall mean a collection of the following items:
A set S of elements called points.
A function, \(\ell\), which assigns to each point x a positive integer \(\ell\)(x) called the level of x.
A relation xRy defined in S, which we read “x is a predecessor of y” or “y is successor of x”. This relation must obey the following conditions:
There is a unique point a1 of level 1. This point we call the origin of the tree.
Every point other than the origin has a unique predecessor.
For any points x, y, if y is a successor of x, then \(\ell \left( y \right) = \ell \left( x \right) + 1\)
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