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Punctuation/numbers - Numbers could be added. The first choice could be ("A-M" OR "N-Z, 1-9") or ("Letters" OR "Numbers"). Punctuation would be hard to accomplish in the tree representation, as punctuation as has no ordering, unlike the alphabet. Punctuation could work well in the list representation though, by simply appending punctuation after "z".
Safety
If in the middle of selecting a letter, a wrong choice is made, Billy could simply hit the "up" key which would move back up a level in the tree. If an entirely wrong letter is input, again hitting the up key from the root node could delete a previous letter.
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