After pseudo-code in
http://www.ics.uci.edu/~eppstein/161/960229.html
Lecture notes by David Eppstein, eppstein@ics.uci.edu
commonSubseqs: find common subsequences
a, b: two sequences
returns
l: list of common subsequences
m: the length of a
n: the length of b
longestCommonSubseq: find the LCS of two sequences
The sequence type must have an __append metamethod. This is
provided by string_ext for strings, and by list for lists.
a, b: two sequences
s: an empty sequence of the same type
returns
s_: the LCS of a and b