localhost ~ # nano /etc/portage/package.unmask
localhost ~ # emerge =x11-proto/xproto-7.0.10
Calculating dependencies... done!

!!! Error: the >=x11-proto/xproto-7.0.6 package conflicts with another package;
!!!        the two packages cannot be installed on the same system together.
!!!        Please use 'emerge --pretend' to determine blockers.

For more information about Blocked Packages, please refer to the following
section of the Gentoo Linux x86 Handbook (architecture is irrelevant):

http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?full=1#blocked