> > But should the map function return lists or ideques?  It seems heavyweight
> > to return ideques that are then just copied and discarded, whereas
> > returning lists seems inconsistent.  That's why I left this out.
>
> I'm thinking of rerturning ideque.  It's (ideque-filter values (ideque-map
> proc ideque ...))
> but saves creating intermediate ideque.

Correct, but does the 'proc' return a list or an ideque?  It has to
return some intermediate sequence data structure, since the whole point
of map-append is for one-to-many mappings.  Returning an ideque is more
consistent, but returning a list is cheaper.