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

Calculating dependencies ... done!
Dependency resolution took 0.87 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "gnome-base/gnome-flashback" has unmet requirements.
- gnome-base/gnome-flashback-3.58.0-r1::gentoo USE="-elogind (-systemd)"

  The following REQUIRED_USE flag constraints are unsatisfied:
    exactly-one-of ( elogind systemd )

(dependency required by "x11-wm/gnome-flashback-herbstluftwm-1.0::gentoo" [ebuild])
(dependency required by "=x11-wm/gnome-flashback-herbstluftwm-1.0" [argument])