ubuntu / # emerge --unmerge xf86-video-nouveau
 * This action can remove important packages! In order to be safer, use
 * `emerge -pv --depclean <atom>` to check for reverse dependencies before
 * removing packages.

 x11-drivers/xf86-video-nouveau
    selected: 0.0.16_pre20101130
   protected: none
     omitted: none

All selected packages: x11-drivers/xf86-video-nouveau-0.0.16_pre20101130

>>> 'Selected' packages are slated for removal.
>>> 'Protected' and 'omitted' packages will not be removed.

>>> Waiting 5 seconds before starting...
>>> (Control-C to abort)...
>>> Unmerging in:  5 4 3 2 1
>>> Unmerging x11-drivers/xf86-video-nouveau-0.0.16_pre20101130...
openpty failed: 'out of pty devices'

ubuntu / # emerge -av xorg-server $(qlist -IC x11-drivers)

These are the packages that would be merged, in order:

Calculating dependencies... done!

emerge: there are no ebuilds built with USE flags to satisfy "x11-libs/libdrm[video_cards_radeon]".
!!! One of the following packages is required to complete your request:
- x11-libs/libdrm-2.4.25 (Change USE: +video_cards_radeon)
(dependency required by "x11-drivers/xf86-video-ati-6.14.1" [ebuild])
(dependency required by "x11-drivers/xf86-video-ati" [argument])