Let Σ be a finite alphabet, Σ n ⊆ Σ be the set of all strings of length n from Σ and Σ * the set of all strings from that alphabet. A string s ∈ Σ * of length n is a sequence of characters that can be indexed as s = s 1 . . . s n . Given a string s ∈ Σ n , we say that u ∈ Σ m is a subsequence of s if there exists a set of indices