-----Description-----
This task requires writing a Lean 4 function that computes the longest common subsequence (LCS) of two input strings.
A subsequence of a string is a sequence that can be derived from the original string by deleting zero or more characters (not necessarily contiguous) without changing the order of the remaining characters.
The function should return a string that is:
1) A subsequence of both input strings.
2) As long as possible among all such common subsequences.

If multiple LCS answers exist (with the same maximum length), returning any one of them is acceptable.

-----Input-----
s1: The first input string.
s2: The second input string.

-----Output-----
A string representing a longest common subsequence of s1 and s2.

