helix user # emerge -a1 emul-linux-x86-medialibs

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

Calculating dependencies... done!
[ebuild     U ] app-emulation/emul-linux-x86-medialibs-10.2 [1.2-r1]
[ebuild     U ] app-emulation/emul-linux-x86-baselibs-10.2 [10.1]
[blocks B     ] <media-video/mplayer-bin-1.0_rc1-r2 (is blocking app-emulation/emul-linux-x86-medialibs-10.2)
[blocks B     ] <app-emulation/emul-linux-x86-medialibs-10.2 (is blocking app-emulation/emul-linux-x86-baselibs-10.2)

!!! Error: The above package list contains packages which cannot be installed
!!!        at the same time on the same system.

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