i read through some answers here and i nodded in agreement with raganwald saying that this only worked as a filter on failure.

but then i read this and i thought "would i really want to give a job to someone who needs a pen and paper to subtract 1 from 255?"

i'm not sure what the lesson is here - perhaps that programming tests are dangerous because it is very easy to over-interpret the results, even when you start with the best of intentions?

[edited for grammar]