On Fri, Sep 19, 2008 at 1:32 AM, Joseph Wright
<[log in to unmask]> wrote:
> I should point out that this was, implicitly, a two-part questions.  Part
> one was about the primitives, part two is how to execute:
>
> \tlp_gset:Nx \g_siunitx_tmpa_tlp {\etex_scantokens:D {#1}}
>
> while _ is active (and I guess ideally : is "other").  In my current setup,
> I only have to worry about @, which I just accept will be a letter.  But I
> have to have _  active.

Ah :) Bit of a pickle.

I guess I'd do something like

\def:Npn \siunitxtmp {
  \tlp_gset:Nx \g_siunitx_tmpa_tlp {\etex_scantokens:D {#1}}}
\char_make_active:N \_
\siunitxtmp

(unless my brain hasn't woken up yet...I think that will work)

Will