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

Calculating dependencies ... done!
Dependency resolution took 0.83 s (backtrack: 0/20).


!!! All ebuilds that could satisfy "=x11-wm/herbstluftwm-9999" have been masked.
!!! One of the following masked packages is required to complete your request:
- x11-wm/herbstluftwm-9999::gentoo (masked by: missing keyword)

For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.