This avoids inserting braces when you type "a_", so you end up typing
"a_i" instead of "a_{i}".
The official LaTeX manual says that braces are correct, but it doesn't
matter for many cases, e.g., when the sub/superscript is just a single
token like "i". It's simply a matter of taste in these cases.
- Frank Mittelbach, a LaTeX maintainer, says that he types "a_i":
https://tex.stackexchange.com/questions/82329/how-bad-for-tex-is-omitting-braces-even-if-the-result-is-the-same#comment176261_82331
- David Carlisle, another LaTeX maintainer, says that a_i "arguably
improves the look of the source code":
https://tex.stackexchange.com/a/82337
- Joseph Wright, yet another LaTeX maintainer, says that he "would
always use braces here, so favour a_{i}":
https://tex.stackexchange.com/a/1929
And I think if it is a matter of taste, then the sane default is to
avoid any magic that gets in the way of users. Anyone who prefers braces
can still enable this argument, but people who do not like them won't
get upset every time they type "_" or "^".
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