I am both accountant (actually educated) and developer. I will not rant (today) on how I disagree with unit testing and there are better ways to get the result.

I am both accountant (actually educated) and developer. I will not rant (today) on how I disagree with unit testing and there are better ways to get the result. I just wanted to point out double bookkeeping analogy is bad. Double bookkeeping is there to make certain types of errors very visible. Double bookkeeping comes from idea that things do not just show up or disappear, but they change state -- value appearing on one account must have come from another. And so double bookkeeping is basically saying -- we want to have a system where it is immediately visible or even impossible to write something down that does not preserve this invariant. It has nothing to do with whether the operation is accounted for correctly. You can still mess up accounts very easily. Double bookkeeping does not mean double checking. You just select two accounts and put the same value on opposite sides of those accounts. This is a single operation. Everything about it can be wrong -- the value can be wrong, the accounts can be wrong and nothing is going to catch that error. Actually, using invariants is one technique I use to ensure my code works correctly. Has nothing to do with unit tests. I just write code, whenever it is possible, in a way that makes it impossible to put it in a wrong state.