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General Update Pattern, II

By adding a key to the data-join, letters that are already displayed are put in the update selection. Now updates can occur anywhere in the array, depending on the overlap between the old letters and the new letters. The text content only needs to be set on enter because the mapping from letter to element never changes; however, the x-position of the text element must now be recomputed on update as well as enter.

It'll be easier to see what's going on when we add animated transitions next!

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