This set is not empty
and it is bounded from above ( since fromiv, 3 is an upper bound of H ).
The Least Upper Bound Principle guarantees the existence of supH. We will
denote it by e. Observe that vn < e for any n, since if vn = e for some n,
then e = vn < vn+1 ≤ e, which implies that e < e and this is a contradiction.