Thank you all who responded.

I suceeded in installing rubygems in /usr/local/bin
by a method along the lines Gregory suggested.
But after that, I switched to the unofficial Debian package
Manuel mentioned.

Now, my original purpose at hand was to install
a command-line argument parser.   By searching
the Internet, I had learned there are getopt, getoptlong,
optparse (parseopt??), and ropt.  (Sorry if I misspelt some
of them).  Reading some documents, I decided I like the
simplicity of ropt.  But then, "gem search opt --remote"
listed getopt, getopt-declare, and OptionParser.
Not ropt.   Argh.

1) Does the gem system work for those libraries which
aren't shown in the listing?  In other words, if the gem
command doesn't show a library, would you need to
install it separately from the gem framework?

2) How do you install ruby libraries which are not on
the gem framework?

3) Which command line argument parser is "standard"?
(Please user your own definition of "standard" :)

Thank you,
Ryo