The **longest common subsequence** (or [**LCS**](http://en.wikipedia.org/wiki/Longest_common_subsequence_problem)) of groups A and B is the longest group of elements from A and B that are common between the two groups and in the same order in each group. For example, the sequences "1234" and "1224533324" have an LCS of "1234":
***1234***
***12***245***3***332***4***
For a string example, consider the sequences "thisisatest" and "testing123testing". An LCS would be "tsitest":
***t***hi***si***sa***test***
***t***e***s***t***i***ng123***test***ing.
Your code only needs to deal with strings.
For more information on this problem please see [Wikipedia](https://en.wikipedia.org/wiki/Longest_common_subsequence_problem).
# --instructions--
Write a case-sensitive function that returns the LCS of two strings. You don't need to show multiple LCS's.
# --hints--
`lcs` should be a function.
```js
assert(typeof lcs == 'function');
```
`lcs("thisisatest", "testing123testing")` should return a string.