A note about if not versus unless.

I am diametrically opposed to tye here. The only win for unless is to make some things be said in a way that more directly matches how we speak. But I would *never* use it for complex expressions. As I have found from painful experience, people do not apply De Morgan's laws on the fly. In other words while debugging it takes a lot of thought to translate:

unless (A or B) {
  ....
}
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and recognize that as

if (!A and !B) {
  ...
}
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After you have been there a couple of times, you learn not to use unless with complex expressions. :-)