Recall that the sequence **b** is a subsequence of the sequence **a**, if **b** can be derived from **a** by removing zero or more elements without changing the order of the remaining elements. For example, if **a** = [1, 2, 1, 3, 1, 2, 1], then possible subsequences of **a** are: [1, 1, 1, 1], [3] and [1, 2, 1, 3, 1, 2, 1], but not [3, 2, 3] and [1, 1, 1, 1, 2].

This is a companion discussion topic for the original entry at https://toph.co/p/alternative-subsequence-the-extension