emerge --sync
emerge -quDN @world

produced the following output:

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

dev-libs/icu:0

(dev-libs/icu-58.2:0/58.2::gentoo, ebuild scheduled for merge) pulled in by
dev-libs/icu:0/58.2 required by (app-office/libreoffice-bin-5.2.4.2:0/0::gentoo, ebuild scheduled for merge)
^^^^^^^

(dev-libs/icu-58.1-r1:0/58.1::gentoo, installed) pulled in by
>=dev-libs/icu-4.4:0/58.1= required by (dev-tex/bibtexu-3.71_p20150521:0/0::gentoo, installed)
^^^^^^^^
(and 12 more with the same problem)

sys-libs/readline:0

(sys-libs/readline-6.3_p8-r3:0/0::gentoo, installed) pulled in by
sys-libs/readline:0/0=[abi_x86_64(-)] required by (dev-db/sqlite-3.15.2:3/3::gentoo, installed)
^^^^^
(and 14 more with the same problem)

(sys-libs/readline-7.0_p1-r1:0/7::gentoo, ebuild scheduled for merge) pulled in by
>=sys-libs/readline-7.0:0= required by (dev-lisp/clisp-2.49-r100:2/7::gentoo, ebuild scheduled for merge)
^^ ^^^^^^

NOTE: Use the '--verbose-conflicts' option to display parents omitted above

The following mask changes are necessary to proceed:
(see "package.unmask" in the portage(5) man page for more details)

1. required by dev-lang/python-2.7.12::gentoo[ncurses,readline]
2. required by dev-python/pyparsing-2.1.10::gentoo[python_targets_python2_7]
3. required by media-gfx/pydot-1.0.28-r2::gentoo
4. required by dev-tex/dot2tex-2.9.0::gentoo
5. required by dev-tex/dot2texi-3.0::gentoo
6. required by app-text/texlive-2015::gentoo[graphics]
7. required by media-gfx/inkscape-0.91-r3::gentoo[latex]
8. required by @selected
9. required by @world (argument)
10. /usr/portage/profiles/package.mask/funtoo-cautionary-oleg:
11. mask rc version of readline. Just for precaution
    =sys-libs/readline-7.0_p1-r1

The following USE changes are necessary to proceed:
(see "package.use" in the portage(5) man page for more details)

1. required by dev-texlive/texlive-xetex-2015::gentoo
2. required by dev-texlive/texlive-formatsextra-2015::gentoo
3. required by app-text/texlive-2015::gentoo[extra]
4. required by media-gfx/inkscape-0.91-r3::gentoo[latex]
5. required by @selected
6. required by @world (argument)
   >=app-text/texlive-core-2015-r1 xetex
7. required by media-gfx/graphviz-2.38.0-r1::gentoo
8. required by media-gfx/pydot-1.0.28-r2::gentoo
9. required by dev-tex/dot2tex-2.9.0::gentoo
10. required by dev-tex/dot2texi-3.0::gentoo
11. required by app-text/texlive-2015::gentoo[graphics]
12. required by media-gfx/inkscape-0.91-r3::gentoo[latex]
13. required by @selected
14. required by @world (argument)
    >=media-libs/gd-2.2.3 fontconfig
15. required by app-text/texlive-core-2015-r1::gentoo[xetex]
16. required by dev-texlive/texlive-xetex-2015::gentoo
17. required by dev-texlive/texlive-formatsextra-2015::gentoo
18. required by app-text/texlive-2015::gentoo[extra]
19. required by media-gfx/inkscape-0.91-r3::gentoo[latex]
20. required by @selected
21. required by @world (argument)
    >=media-libs/harfbuzz-1.4.1 graphite

Use --autounmask-write to write changes to config files (honoring
CONFIG_PROTECT). Carefully examine the list of proposed changes,
paying special attention to mask or keyword changes that may expose
experimental or unstable packages.

emerge: there are no ebuilds built with USE flags to satisfy ">=app-text/texlive-core-2010[xetex]".
!!! One of the following packages is required to complete your request:

• app-text/texlive-core-2015::gentoo (Change USE: +xetex)
  (dependency required by "dev-texlive/texlive-xetex-2015::gentoo" [ebuild])
  (dependency required by "app-text/texlive-2015::gentoo[xetex]" [ebuild])
  (dependency required by "media-gfx/inkscape-0.91-r3::gentoo[latex]" [installed])
  (dependency required by "@selected" [set])
  (dependency required by "@world" [argument])

I went to irc.freenode.org to discuss on #funtoo how to solve those conflicts, and thought I should provide more and better information. So I ran, without doing anything else in the terminal in between:

emerge -quDN @world --verbose-conflicts

That installed everything, without conflicts, without any USE flag changes necessary, without any masking necessary, just everything is fine and solved.

It appears that --verbose-conflicts solved all the conflicts.