$ sudo emerge -uDNav world

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

Calculating world dependencies... done!
[ebuild     UD] dev-lang/tcl-8.4.14 [8.5.0] USE="-debug% -threads" 3,462 kB 
[ebuild     UD] dev-lang/tk-8.4.14 [8.5.0] USE="-debug% -threads" 3,192 kB 

Total: 2 packages (2 downgrades), Size of downloads: 6,654 kB

Would you like to merge these packages? [Yes/No]