Currently, provision of scratch variables (such as \l_tmpa_tl) is
somewhat inconsistent in expl3. However, it is also not entirely clear
if these 'general scratch' variables are really idea in any case. Do
people see a general need for these variables, or will most users expect
to define their own scratch space in all cases?
(The kernel does not use any of these scratch variables, so they are
only intended for third-party use in any case.)