Joseph Wright writes:
 > >> A quick search shows they are used in the tl module. I'd stick to defining "private" scratch variables.
 > >
 > > Should we change this?
 > > I.e., use \l_kernel_tmpa_tl or \l_reserveda_tl instead for us and let package writers use \l_tmpa_tl ?
 > >
 > > -- Will
 > >
 > I think we probably should: the documentation seems to imply that they 
 > are available, and it would be best if there was clear demarcation.

yes, keep private kernel scratch registers only used in controlled situations
and documented as not being available for programming - just like we did with
\reserved@a etc in 2e

\l_kernel_tmpa_tl sounds  fine enough for those

frank