>I understood that the search option was not available for non-registered members, but I would have thought that the ability to walk backward on the article tree/map was a basic necessity for any conferencing program....

Not "conferencing" but "threaded messages".

I would be interested to know how many threaded messages system that will allow you to get the map of prior messages that we have already read.

AFAIK, you can't go back in time for more than 2 weeks in CIS, can't get the map for messages you already read in Newsgroups and this is not available in Listserv as well.

Here, there's no limit to what far you want to retrieve older messages map , available for PUTM.