vi '_' command

[Dave Decot]

What does the undocumented vi 3.7 command '_' (underline) mean?  What are its
implications?  It seems to be a synonym for 0 or ^ or something,
but I haven't figured out why you would want it.

And what is the :ab command?  Seems to allow abbreviations, but I don't have
any documentation on it.  Nobody answered Jim Davis's question about the ~
(capitalize one character) command, either.  My manual says both of these
characters are "Unused."

Dave Decot	(No address, moving to CA)