virtual problem

I have not been able to do an "emerge --update world" for a couple of
weeks now due to various vritual/?????????? packages.  e.g., xmms
depended on virtual/opengl.  I unermerged it and ended up emergeing a
number of other packages in the process of getting it back.  Now it is
done one of those packages, qt, complains of missing virtual/glu when I
try and emerge world.

Can someone explain this whole virtual thing and how to fix, or give a
reference in the docs?

BillK

* I have noted a couple of other emails along these lines, but havent
seen any fixes yet.