this starts to become as cryptic as the previously mentioned 'cryptic regex.'

cryptic is anything that can stay in the head 'ram' of a normal programmer. stuff that typically needs to be written down to make sense.

i went through the above code with pencil and paper and it made sense. now i can look at the code and it makes sense. the person who originally wrote the code had to go through those steps too (not necessarily on paper, but loading the process into head ram). same with regex.

no big deal. i know about slices, and if i didn't that's what mentors are for---oh, that symbol? search for python slices! or a python book. at least python doesn't has less than a handful nonsearchable crypticness.

the comment linking to the eratosthenes sieving explanation was helpful.