# emerge -DNuav @world --verbose-conflicts --backtrack=30

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

Calculating dependencies       ... done!                                                  A                                 ^                                   ^                           ^                      B                        B              ,    p
!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

dev-lang/perl:0

  (dev-lang/perl-5.24.0:0/5.24::gentoo, ebuild scheduled for merge) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

  (dev-lang/perl-5.20.2-r1:0/5.20::gentoo, ebuild scheduled for merge) pulled in by
    dev-lang/perl:0/5.20=[-build(-)] required by (dev-perl/XML-Parser-2.440.0:0/0::gentoo, installed)
                 ^^^^^^^^
    dev-lang/perl:0/5.20=[-build(-)] required by (dev-perl/PlRPC-0.202.0-r2:0/0::gentoo, installed)
                 ^^^^^^^^

dev-libs/boost:0

  (dev-libs/boost-1.61.0:0/1.61.0::gentoo, ebuild scheduled for merge) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

  (dev-libs/boost-1.60.0:0/1.60.0::gentoo, ebuild scheduled for merge) pulled in by
    dev-libs/boost:0/1.60.0= required by (dev-util/mdds-0.12.1:0/0.12.1::gentoo, installed)
                  ^^^^^^^^^^

dev-util/boost-build:0

  (dev-util/boost-build-1.61.0:0/0::gentoo, ebuild scheduled for merge) pulled in by
    =dev-util/boost-build-1.61* required by (dev-libs/boost-1.61.0:0/1.61.0::gentoo, ebuild scheduled for merge)
    ^                     ^^^^^                                                                                                                                

  (dev-util/boost-build-1.60.0:0/0::gentoo, installed) pulled in by
    =dev-util/boost-build-1.60* required by (dev-libs/boost-1.60.0:0/1.60.0::gentoo, ebuild scheduled for merge)
    ^                     ^^^^^                                                                                                                                

media-libs/glew:0

  (media-libs/glew-1.13.0:0/1.13::gentoo, ebuild scheduled for merge) pulled in by
    (no parents that aren't satisfied by other packages in this slot)

  (media-libs/glew-1.10.0-r2:0/1.10::gentoo, ebuild scheduled for merge) pulled in by
    media-libs/glew:0/1.10= required by (games-arcade/supertux-0.4.0:0/0::gentoo, installed)
                   ^^^^^^^^


It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.