rcs, version 5.7.0

Adapted from the man page:

The rcs Revision Control System manages multiple revisions of files.
rcs automates the storing, retrieval, logging, identification, and merging
of revisions.  rcs is useful for text that is revised frequently, for example
programs, documentation, graphics, papers, and form letters.

The basic user interface is extremely simple.  The novice only needs
to learn two commands:
        ci      (check in)
and
        co      (check out)

to store and retrieve multiple revisions of text.  rcs saves all old revisions
in a space efficient way.  Changes no longer destroy the original, because the
previous revisions remain accessible.  Revisions can be retrieved according to
ranges of revision numbers, symbolic names, dates, authors, and states.

rcs logs all changes automatically.  Besides the text of each revision, rcs
stores the author, the date and time of check-in, and a log message 
summarizing the change.  The logging makes it easy to find out what 
happened to a module, without having to compare source listings or having 
to track down colleagues.