Hi, I've been thinking more about this. Just so we're clear, what you're looking for is a function that can be given a token list and return true or false depending if, without expansion, the list contains a quark. So: \def\abc{\q_stop} \if_contains_quarks_p:n{hello\abc} -> 'false' \if_contains_quarks_p:n{hello\q_stop} -> 'true' Right? No matter how this function is implemented, it's always going to be slow. One approach might be (in LaTeX2e pseudocode): \edef\quarktest{\detokenize{\q_}} \@ifin{\quarktest}{\detokenize{#1}} \in@ But it's not necessarily robust. Maybe some punk will come along and define a whole bunch of \q_ macros that aren't quarks. Or define quarks that don't start with \q_. Or maybe you want to distinguish the case when a quark has been misspelled. Anyway, an alternative might be to globally record all quarks are they're defined: \def_new:Npn \quark_new:N #1{ \seq_push:Nn \g_all_quarks_seq {#1} \tlp_new:Nn #1{#1} } Then \g_all_quarks_seq can be queried to answer the question "is this token/control sequence actually a quark?". Not fast, but it's more bullet-proof. - What do you think? - How necessary is it to be able to answer this question in general? (I.e., when you're not looking for a *specific* quark.) Will