> in the hope that it may be useful, I think that the following is
> something that is mathematically/semantically correct but that
> produces similar problems:
>
> \documentclass{article}
> \begin{document}
> $R^{a[bc}G^{cb]g}$
> \end{document}
>
It also works with `mathml-` option. One issue I found with this
option is that parenthesis don't have correct size, so I guess that it
is still right thing to do to fix the TeX code.
>
> I think that the following should give a sound xml:
>
> \documentclass{article}
> \begin{document}
> $R^{a\left[bc\right.}G^{\left.cb\right]g}$
> \end{document}
Yes, it seems to produce good result, both visual and xml.
Best regards,
Michal
