Another (sort of) related question.  I'm try to add something to an
existing delimited macro.  Given an initial macro:

\def:Npn \my-macro #1\q_stop {Stuff~here}

I can do:

\toks_set:Nn \l_tmpa_toks \exp_after:NN \exp_after:NN \exp_after:NN
{\cs:w my-macro \cs_end: ##1 \q_stop}

\def:cpn {my-macro} #1\q_stop {\toks_use:N\l_tmpa_toks Added~stuff}

to add things on.  However, I again wonder if I'm missing a better way
(I tried things with \exp_not:Nf and so on, but failed).
-- 
Joseph Wright