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A few gotchas here:

* "seq" is usually an external process. You may want to do it in bash's arithmetic for, use a while loop with `$(( ... ))", or write a `seq` function (you still need to spawn a subshell for command expansion though.)

* "(( ... ))" is not in POSIX shell, but "$(( ... ))" is. You should be able to get around with some [ "$(( blah ))" -eq 0 ] tests.

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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.

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