desktop ~ # cat /etc/portage/profile/package.provided
sys-kernel/gentoo-sources
desktop ~ # emerge -pvt nvidia-kernel
Invalid package name in package.provided: sys-kernel/gentoo-sources

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

Calculating dependencies... done!
[ebuild  N    ] media-video/nvidia-kernel-1.0.8756  0 kB
[ebuild  N    ]  sys-kernel/gentoo-sources-2.6.16-r2  USE="symlink -build -doc" 166 kB

Total size of downloads: 166 kB