Error when using super/subscript embellishments

[jdujava]

Command definition macros in https://ctan.org/pkg/usrguide (\NewDocumentCommand) support so called "embellishments".
One can choose for such an embellishment ^ or _, for example

\NewDocumentCommand{\cder}{s e{_}}{
    \IfBooleanT{#1}{\mspace{-3mu}}
    \bm{\nabla}\IfValueT{#2}{_{\mspace{-5mu}#2}\mspace{2mu}}
}

This however results in error, since as of now, grammar requires curly_group/letter/command name after ^/_.

Having following curly_group/letter/command be optional would seem to fix the issue. I just want to ask, if this would be acceptable (will post PR in that case), or there can be some side-effects, or perhaps more efficient fix.

[jdujava]

This is turning out harder than I thought, since naive implementation (making curly_group/letter/command optional, with superscript/subscript node having right associativity) causes regressions similar to #132. If I understand correctly, this is due to lexical precedence of word compared to letter..