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

Calculating dependencies ... done!
Dependency resolution took 0.88 s (backtrack: 0/20).


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by games-puzzle/quadrapassel-49.3::gentoo
# required by =games-puzzle/quadrapassel-49.3 (argument)
>=media-libs/gsound-1.0.3 vala

!!! The ebuild selected to satisfy "media-libs/gsound[vala]" has unmet requirements.
- media-libs/gsound-1.0.3::gentoo USE="-gtk-doc -introspection -vala"

  The following REQUIRED_USE flag constraints are unsatisfied:
    vala? ( introspection )

(dependency required by "games-puzzle/quadrapassel-49.3::gentoo" [ebuild])
(dependency required by "=games-puzzle/quadrapassel-49.3" [argument])