A tree is a tree-like non empty set. In the sequel T , T 1 denote trees. If x T , then x is a finite sequence of elements of . Let us consider T . We see that the element of T is a finite sequence of elements of .
T holds q T .
T and T . T T 1 is a tree. T T 1 is a tree. Let us observe that there exists a tree which is finite. Let us consider n .