>> \def\quark\quark{\quark}
> with error message "Use of \quark doesn't match its definition" if it's ever
> accidentally executed. I *think* this definition still fulfils the use case
> for quarks themselves, but I wonder if it would involve a little too much
> code shuffling to be worth implementing for expl3.
> (And it makes it more difficult to peek inside a quark, if that's ever
> necessary.)
> Before I forget about this idea, does anyone have any comments on the
> matter?

Hi Will,

if I understand correctly, quarks are meant to work in cases like

\ifx\foo\quark  ... \else ... \fi

This requires quarks to be defined as expanding to themselves.
Or it requires to distinguish between a quark and a token list containing
that quark.

I personally don't use quarks as anything else than delimiters, so any
definition is fine, but I guess that property of quarks is used in

\tl_set:Nn \foo {#1}
\quark_if_recursion_tail_stop:N \foo


\prop_get:NnN \foo {key} \bar
\quark_if_no_value:NTF \bar { } { }

This could be solved by altering quark tests, perhaps...?