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

Calculating dependencies ...

!!! Problem resolving dependencies for =gnome-base/gnome-flashback-3.56.0
 done!
Dependency resolution took 0.82 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "=gnome-base/gnome-flashback-3.56.0" has unmet requirements.
- gnome-base/gnome-flashback-3.56.0::gentoo USE="-elogind (-systemd)"

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