Another question I noticed is whether mappings should be made nestable
or not. I _think_ that \seq_map_function:NN is nestable, and that
\seq_map_inline:Nn and \seq_map_variable:NNn are not. All three can be
made expandable, I believe. Should they?

> Or, much preferred from my point of view, to add some seq-transformation
> functions such as
>     \seq_to_clist:N
> or
>     \clist_set_from_seq:NN
> which seems to be what's happening above. In fact, this latter function is
> already present in l3candidates awaiting possible inclusion into l3seq.

Sorry, I was not thinking straight. I would prefer \clist_(set_)from_seq:NN.

A modus operandi to easily switch to the proposed l3seq would be first
to get rid of all the explicit \seq_elt:w ... \seq_elt_end:, for
instance by replacing \seq_show:N by \seq_display:N in testfiles, and
replace ad hoc constructs by some \..._map_inline where possible.

If something slightly more advanced (e.g. some things in Will's
version of the NFSS) is needed, I'm happy to code two versions:
- with \seq_elt:w ... \seq_elt_end: now, so that everything is in
l3seq (or l3candidates)
- with \seq_elt:n for later when/if we switch.

-- Bruno