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Wouldn't that change the order of elements?

The only guarantee i saw was that the output array did not contain the rejected elements. (and looking at the reference docs, that's all it says)

It appears order is guaranteed to be maintained but it's not specified :)

No. [1,2,3].reject! { |x| x == 1 } would give you [3,2], rearranging the order of elements. It would work for sets.

maybe there's a finalization step that's necessary to free up memory? This seems like the most straightforward solution though