On 16/11/2021 14:43, Ihor Radchenko wrote:
> Max Nikulin writes:
> 
>> Better docs and some restriction on defcustom values were discussed earlier:
>> https://list.orgmode.org/87k0oyd3pw.fsf@nicolasgoaziou.fr/
>> Nicolas Goaziou. Re: Using backticks for the inline code delimeter? Mon,
>> 19 Apr 2021 11:27:07 +0200
>>
>> Sorry, I have not prepared a patch. I am not confident with defcustom
>> fine tuning and have not experimented with it since that time.
> 
> Maybe something like the attached?

My draft version is attached. Ihor, thank you for inspiration. Feel free 
to improve it. I hope, it makes problem more apparent to user who tries 
to customize markers. Have I missed some undesired side effects?

Unfortunately with "set" type in defcustom "catch inappropriate" item is 
not hidden for valid configuration.

I do not mind if you commit any variant.