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

Calculating dependencies ...

!!! Problem resolving dependencies for =gnome-base/gnome-flashback-3.54.0-r1
 done!
Dependency resolution took 0.83 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "=gnome-base/gnome-flashback-3.54.0-r1" has unmet requirements.
- gnome-base/gnome-flashback-3.54.0-r1::gentoo USE="-elogind (-systemd)"

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