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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 or list. If an entirely wrong letter is input, hitting the up key from the root node could delete a previous letter. Deleting a letter with the "up" key might not be very learnable, and we might have to devise a way to show that.
Learnability
The tree representation might be confusing to understand, but we believe it would be fairly obvious to pick up (assuming Billy knows the order of the alphabet). Deleting an entered letter or moving up a level in the tree also seems fairly intuitive, but will need to be tested.
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