I've been trying to work out if the current expl3 has anything like
\patchcmd from etoolbox.  Will suggested looking at the various \tlp_
... macros.  However, even a simple two-part test in this way fails:

    % Do something
    % Also do something

I guess I'm abusing the functions.  So have I missed something obvious?
 The ability to patch existing functions is very handy.
Joseph Wright