This also illustrates another problem: all the programmers here are assuming perfect domain knowledge even exists in the first place. When it doesn't, Excel wi

This also illustrates another problem: all the programmers here are assuming perfect domain knowledge even exists in the first place. When it doesn't, Excel will work fine (and will only cause disaster in individual cases). A "proper" unit-tested solution will either not work at all or just won't support half the use cases. And without someone specifying the domain knowledge, there's nothing any programmer, no matter how well-versed in unit testing, agile, or ... can do about it.