Define macro that scans ahead for super- and/or subscript, “absorbs” them and moves their argument

Question

I have a lot of mathematical macros that already have a super- and/or subscript by definition, i.e. something like

\newcommand*{\mymathsym}{x^{\text{foo}}_{\text{bar}}}

In the text body, these symbols frequently need an extra sup-/subscript. This means, the author must remember to put the main symbol into an pair of {}-braces, otherwise a double sup-/subscript error occurs.

\begin{equation}
  {\mymathsym}^{\text{extra}}
\end{equation}

The extra superscript becomes a secondary superscript and it set slightly higher and smaller: This has two drawbacks: a) In the special field of application, both superscripts stand on the same hierarchy level from a conceptional viewpoint. In other words, both superscripts should actually be printed as a list "foo, extra" and the opposed order "extra, foo" would be equally good. b) If the primary sup- and subscript are very unbalanced in length, the secondary superscript is set far apart, e.g.

\newcommand*{\mymathsymlong}{x^{\text{foo}}_{\text{very long foobar}}}

and

\begin{equation}
{\mymathsymlong}^{\text{extra}}
\end{equation}

yields

As a workaround, I currently use the following definition that takes an optional argument and appends the argument to the internal superscript:

\newcommand*{\mymathsymext}[1][]{x^{\text{foo}\if!#1!\else, #1\fi}_{\text{very long foobar}}}

(N.b. I know that the condition \if!#1! is not the correct way to test for an empty argument, because it fails if the argument expands to a !. But I think you get the idea what the macro does.)

It is used as

\begin{equation}
\mymathsymext \qquad\text{vs.}\qquad \mymathsymext[\text{extra}]
\end{equation}

and yields However, this has two major drawbacks: a) \newcommand only supports a single optional argument. Hence I need to decide at design time, if either an additional superscript or an additional subscript might eventually be needed. I cannot support both. b) The user has to remember an unusual syntax for placing additional sup-/subscripts.

Question:

How does one define a macro \mymathsymsuper that

• scans ahead, if it is followed by a supscript character ^<tok> and/or subscript character _<tok> each followed by an additional token <tok>
• "absorbs them", and
• moves <tok> to end of its internal sup-/subscript separated by a comma?

Full MWE:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\newcommand*{\mymathsym}{x^{\text{foo}}_{\text{bar}}}

\newcommand*{\mymathsymlong}{x^{\text{foo}}_{\text{very long foobar}}}

\newcommand*{\mymathsymext}[1][]{x^{\text{foo}\if!#1!\else, #1\fi}_{\text{very long foobar}}}

\begin{document}

Here, the author must know that \verb#\mymathsym# has already a super- and subscript and must remember to put the main symbol into a pair of \{\}-braces, otherwise a double sup-/subscript error occurs.
The extra superscript becomes a secondary superscript and it set slightly higher and smaller:
\begin{equation}
{\mymathsym}^{\text{extra}}
\end{equation}

If the primary sup- and subscript are very unbalanced in their length, the secondary subscript is set very far apart:
\begin{equation}
{\mymathsymlong}^{\text{extra}}
\end{equation}

This extended macro takes an optional argument and ``absorbs'' the extra superscript into the primary superscript:
\begin{equation}
\mymathsymext \qquad\text{vs.}\qquad \mymathsymext[\text{extra}]
\end{equation}
Still, the author must remember this ``unusual'' syntax and it only supports either an extra super- or subscript, bot not both.

\paragraph{Question:}
How does one define a macro \verb#\mymathsymsuper# that
\begin{itemize}
  \item scans ahead if it followed by a superscript character $\verb!^!\langle \mathit{token}_\text{sup}\rangle$ and/or subscript character $\verb!_!\langle \mathit{token}_\text{sub}\rangle$ each followed by an    additional token $\mathit{token}_\text{sup}$ and $\mathit{token}_\text{sub}$ resp.
  \item ``absorbs them'', and
  \item moves $\mathit{token}_\text{sup}$ and/or $\mathit{token}_\text{sub}$ to end of its internal sup-/subscript separated by a comma?
\end{itemize}

\end{document}

Answer 1

It's a breeze with xparse:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}

\NewDocumentCommand{\mymathsym}{e{^_}}{%
  x^{\mathrm{foo}\IfValueT{#1}{,#1}}_{\mathrm{bar}\IfValueT{#2}{,#2}}%
}

\begin{document}

\begin{gather}
\mymathsym \\
\mymathsym^{\mathrm{extrasup}} \\
\mymathsym_{\mathrm{extrasub}} \\
\mymathsym^{\mathrm{extrasup}}_{\mathrm{extrasub}} \\
\mymathsym_{\mathrm{extrasub}}^{\mathrm{extrasup}}   
\end{gather}

\end{document}

With e{^_} the macro looks ahead for ^ or _ (in either order) and assigns #1 to the superscript, #2 to the subscript. You can test for the presence with \IfValueT (or \IfValueTF if you want to do some action with an absent sub/superscript).