Hi,

I want to make a proper definition of a new command to be able to
process it in Lua. I found something on StackOverflow [1] but that
breaks in enumeration _and_ when interaction is enabled and I do not
understand why? Can you give me some help here?

I want to make commands like \foo{bla} and \foo[b]{bla} possible at the
same time. Here is a minimal example (also attached):

```
\startluacode
function userdata.my_command(keywords, text)
	commands.writestatus("test", string.format("text: %s, config: %s", text, keywords))
    local cfg = utilities.parsers.settings_to_hash(keywords)

	if cfg['emph'] ~= nil then
		context("\\emph{" .. text .. "}")
	elseif cfg['bold'] ~= nil then
		context("\\bold{" .. text .. "}")
	else
		context(text)
	end
end
\stopluacode

% without that line there is no problem
\setupinteraction[state=start]

\defineenumeration[enu][text=Enu]

\unprotect
\def\mc{\dosingleempty\do_my_command}
\def\do_my_command[#1]#2{%
 	\iffirstargument{%
		\ctxlua{userdata.my_command('#1', [==[#2]==])}%
	}\else {%
		\ctxlua{userdata.my_command('', [==[#2]==])}%
	}\fi
}
\protect

\starttext

% all the following works
\mc{foo} \mc[bold]{foo} \mc[emph]{foo}

\startenu[title=Some]
\stopenu

% this breaks
\startenu[title=Some \mc{foo}]
\stopenu

\stoptext
```

The compilation fails with:
```
tex error       > tex error on line 41 in file ./test.tex: Use of \do_my_command doesn't match its definition
```

Best,
Gerion

[1] https://tex.stackexchange.com/a/55778