virtual package? aptitude says "not a real package".

[furufuru@ccsr.u-tokyo.ac.jp]

Hello all,

aptitude sometimes says some package is "not a real package".
For instance:

    $ aptitude show emacs-dl-wnn
    Package: emacs-dl-wnn
    State: not a real package

When I try to install such a package using "aptitude install",
nothing is installed.  Could someone tell me what this is?

I remember that when I was using woody, I sometimes installed
a "virtual package", whose sole purpose was to automatically
install the other packages on which it nominally "depends".
At first I thought this "not a real package" business was just
that.  But, I'm puzzled because such a package installs
nothing. . . .

Oh, and I'm now using etch (testing).

Regards,
Ryo