We therefore need to locate shared motifs that are separated across exons, which means
that the motifs are not required to be contiguous. To model this situation, we need
to enlist subsequences.
Problem
A string u is a common subsequence of strings s and t if the symbols of u
appear in order as a subsequence of both s and t.
For example, "ACTG" is a common subsequence of "AACCTTGG" and "ACACTGTGA".
Analogously to the definition of longest common substring,
u is a longest common subsequence of s and t if there does not exist
a longer common subsequence of the two strings. Continuing our above example, "ACCTTG" is a
longest common subsequence of "AACCTTGG" and "ACACTGTGA", as is "AACTGG".