Multiple User Source Code Managers ?

[Peter Silva]

I'm looking for a portable source code manager that lets multiple users
modify the same file at the same time.  Two+ users should be able to get
the same mod file (via command like SCCS "get"), make changes, and only
know that someone else was working on the same file if the two happen to
modify the same LINES of code.

In that case, an error message would be given when the second modifier of
the lines tried to return his mod (using a command like SCCS 'delta').

The manager should also maintain multiple variations of a single package.
The most obvious example of a 'variation' is for hardware dependencies,
but there are many others. We would like to be able to make changes that
affect a specific number (one, some, all) variations.  

RCS and SCCS meet neither of these requirements. Public domain, shareware
and commercial software are all of interest.   

I have read a paper about SUN's translucent file system which sounded
interesting, and elegantly supports multiple versions.  I don't remember
it dealing with concurrent editing.  

I've recently learned of the freeware 'cvs', as well.

Experiences, recommendations, etc... wanted.  Will post result if
sufficient interest shown.

Peter Silva
psilva at cid.aes.doe.ca
(514) 421-4692