Alignment of subscripts and superscripts that employ mathchoice

Question

I have defined a macro which, for the purposes of demonstration, I have stripped down to the following:
newcommand{sub}[1]{mathchoice{_{#1}}{}{}{}}

However, with this macro the subscript no longer plays nicely with superscripts. That is to say, depending on the order in which the two are applied, one of them does not align to the right edge of the base symbol. Consider, for example, xsub{i}^{2} and x^{2}sub{i}:

I'd like to get the canonical behavior where both subscript and superscript are basically flush up against the x:

David Carlisle · Answer

You should move the _ out of the mathchoice.
newcommand{sub}[1]{_{mathchoice{}{}{scriptstyle#1}{scriptscriptstyle#1}}}

I left the first two branches empty as they will not be taken as the style inside the _{...} will be scriptstyle or scriptscriptstyle depending on the nesting level, never display or text style.

Alignment of subscripts and superscripts that employ mathchoice

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