A decreasing subsequence of a permutation is a subsequence
of the permutation whose elements decrease. For the permutation $\pi$ equal to (8, 2, 1, 6, 5, 7, 4, 3, 9),
(8, 7, 3) and (8, 6, 5, 4, 3) are decreasing subsequences. You may verify that the second subsequence is the
longest possible decreasing subsequence of $\pi$.
