`DUP n` is 1-indexed
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DUP 0 is rejected, whereas DUP 1 is equivalent to DUP.
It would be more intuitive to accept DUP 0 as equivalent to DUP, and have DUP n duplicate the nth stack element, where the top stack element is the 0th element.
Link items together to show that they're related or that one is blocking others.