Will Robertson wrote:
> P.S.  I guess that something like the above \EditInstance would require
> a saved keyval list for the parameters used to generate the instance;
> however, I don't think that's an unacceptable use of memory.

I'm not sure it would. What creating an instance actually does is
convert all of the keys into their basic tasks (such as \tl_set:Nn),
then store them.  I think it would be possible to get this "distilled"
information back from the instance, add new items to it and rebuild the
instance without needing the original keys.  All of the instances look like

\TP_push_assignments:n { <distilled keys> } \\TP>"type"/"instance"

so we just have to have the correct delimited macro to get the
<distilled keys> back out. The internal macro \\TP>"type"/"instance"
does not contain any of the settings, just their use.
-- 
Joseph Wright