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    $\begingroup$ Arguably the better method, because this one doesn't require modification to the Turing Machine's set of allowable symbols, which effectively makes it into a different Turing Machine. $\endgroup$ Commented Aug 10, 2021 at 8:58
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    $\begingroup$ @The_Sympathizer but it does have the drawback of requiring an additional $O(n)$ space. And anyways it still needs some additional symbol as a "separator". This answer is still very good considering that it is more practical :) $\endgroup$ Commented Aug 10, 2021 at 11:55
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    $\begingroup$ @nirshahar worrying about space overhead is silly when the topic is Turing machines – they have infinite memory... — If you're going to discuss anything resembling real-world performance, then you should just allow for two registers that can store the positions as addresses, instead of encoding them in unary on the tape. $\endgroup$ Commented Aug 10, 2021 at 14:46
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    $\begingroup$ How does the Turing machine create the copy? I'm wondering about how you indicate where in the input you are while copying -- can this be done without introducing new symbols like @The_Sympathizer says? Maybe you overwrite the symbol you're copying with $\sqcup$? $\endgroup$ Commented Aug 10, 2021 at 22:04
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    $\begingroup$ I accepted the other answer because this answer seems to mask its elegance behind "creating a copy of the input." Like Kyle mentions, I think some kind of marking procedure or some more complicated state representation would be required to actually create a copy. $\endgroup$ Commented Aug 10, 2021 at 22:42