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

Calculating dependencies ...

!!! Problem resolving dependencies for =sys-apps/duc-1.4.5
 done!
Dependency resolution took 0.83 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "=sys-apps/duc-1.4.5" has unmet requirements.
- sys-apps/duc-1.4.5::gentoo USE="X cairo ncurses sqlite tokyocabinet -gui -leveldb"

  The following REQUIRED_USE flag constraints are unsatisfied:
    exactly-one-of ( tokyocabinet leveldb sqlite ) X? ( gui )

  The above constraints are a subset of the following complete expression:
    exactly-one-of ( tokyocabinet leveldb sqlite ) X? ( cairo gui )