Yes you're right that (( )) isn't POSIX but $(( )) is -- I was lazy and borrowed it from the example. The one change I made was to change brace expansion {1..1

Yes you're right that (( )) isn't POSIX but $(( )) is -- I was lazy and borrowed it from the example. The one change I made was to change brace expansion {1..100} to $(seq 100) because the former isn't in POSIX, but yeah the whole thing wasn't POSIX anyway. But it doesn't really detract from the point -- if shell has arithmetic and bitwise operators built in, it's not a stretch for it to have set operations built in.