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http://dev.gentoo.org/~kosmikus/darcs.eclass
http://dev.gentoo.org/~kosmikus/darcs-darcs-0.9.17.ebuild

You should put the files in a local portage overlay. (If you don't know
what that is, look for PORTDIR_OVERLAY in the portage documentation.
-- I am assuming you have an overlay in /usr/local/portage from now
on.) They have to go to the following locations:

/usr/local/portage/eclass/darcs.eclass
/usr/local/portage/dev-util/darcs-darcs/darcs-darcs-0.9.17.ebuild

Then, say

cd /usr/local/portage/dev-util/darcs-darcs
/usr/sbin/ebuild ./darcs-darcs-0.9.17.ebuild digest

After that, the ebuild is installed, and can be normally emerged
using

ACCEPT_KEYWORDS="~x86" emerge darcs-darcs

IMPORTANT:
This ebuild installs the "darcs" binary to

/usr/bin/darcs-darcs

It overwrites any other files from a normal darcs installation,
though. 

Have fun,
  Andres