Vertical alignment of left/right subscripts

Question

Using mathtools's \prescript, the subscripts in $\prescript{}{n}X_m$ are not vertically aligned. The problem does not seem to appear when using ${}_n X_m$, but this is undesirable for other reasons.

It seems \prescript does not take into account empty sub/superscripts, like TeX's mechanism does?

Of course I can do $\prescript{}{n}X^{\mbox{}}_m$, but I'd rather have the n up than the m down...

Answer 1

It's a bit more complicated than \prescript can deal with. One should add a “phantom” superscript on the other side, if one superscript is present.

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\xprescript}{e{_^}me{_^}}{%
  \makexprescript{#1}{#2}{#3}{#4}{#5}%
}

\ExplSyntaxOn
\NewDocumentCommand{\makexprescript}{mmmmm}
 {
  \bool_lazy_and:nnTF { \tl_if_novalue_p:n { #2 } } { \tl_if_novalue_p:n { #5 } }
   {% there is neither a presuperscript nor a postsuperscript
    \vphantom{#3} % the nucleus for the prescripts
    \tl_if_novalue:nF { #1 } { \sb{#1} } % the presubscript
    #3 % the nucleus
    \tl_if_novalue:nF { #4 } { \sb{#4} } % the postsubscript
   }
   {% there is either a presuperscript or a postsuperscript
    \vphantom{#3} % the nucleus for the prescripts
    \sp{\tl_if_novalue:nF { #2 } { #2 } }
    \tl_if_novalue:nF { #1 } { \sb{#1} } % the presubscript
    #3 % the nucleus
    \sp{\tl_if_novalue:nF { #5 } { #5 } }
    \tl_if_novalue:nF { #4 } { \sb{#4} } % the postsubscript
   }
 }
\ExplSyntaxOff

\begin{document}

$\xprescript_n{X}_m + \xprescript_n^2{X}_m + \xprescript_n{X}_m^2$

\end{document}