[gentoo-user] How do you deal with overlays and emerge -u world?

[Nikos Chantziaras]

There are quite a few overlays that offer helpful ebuilds.  kde-testing 
for example is one of them.  However, I don't use it (and others) 
because I can't find a way of stopping the hordes of chaos that appear 
when doing "emerge -u world".  Is there really no way to somehow deal 
with this?  Right now the only thing I can do is add the overlay, copy 
the ebuild I want to my local overlay and then remove the overlay again. 
  Any alternatives to this approach?