Somehow I doubt that his existing code has unit test code with it. So just what is it supposed to do? Mudding the difference between what it's defined to do vs what it just happens to do is one of my pet issues, too.

In this case, I'd probably try to test against what the old code actually does, especially if it's been in production for a while (with people depending on its quirks). Exercise each feature of the old code (and any combinations that seem to be interdependent), capture the output, and diff. (Not nearly as easily done as said, I know. :-( )

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The hell with paco, vote for Erudil!
:wq