一般情形下，我都只启用security源进行升级。但是其中有一次，好像是为了安装calibre软件，我临时打开了update源，安装了该软件，导致libqtcore4等一系列软件包的版本高于security源的版本。
于是问题就出来了
要安装calligra套装，结果
The following packages have unmet dependencies:
calligra : Depends: karbon (>= 1:2.4.0-0ubuntu2.1) but it is not going to be
installed
Depends: calligraflow (>= 1:2.4.0-0ubuntu2.1) but it is not going
to be installed
Depends: calligraplan (>= 1:2.4.0-0ubuntu2.1) but it is not going
to be installed
Depends: calligrastage (>= 1:2.4.0-0ubuntu2.1) but it is not going
to be installed
Depends: krita (>= 1:2.4.0-0ubuntu2.1) but it is not going to be
installed
Depends: calligrasheets (>= 1:2.4.0-0ubuntu2.1) but it is not going
to be installed
Depends: calligrawords (>= 1:2.4.0-0ubuntu2.1) but it is not going
to be installed
Depends: kthesaurus (>= 1:2.4.0-0ubuntu2.1) but it is not going to
be installed
Depends: kexi (>= 1:2.4.0-0ubuntu2.1) but it is not going to be
installed
Depends: braindump (>= 1:2.4.0-0ubuntu2.1) but it is not going to
be installed
E: Unable to correct problems, you have held broken packages.
倒腾好半天，才发现是libqt系列库在磁盘上的已安装版本高于security源的版本（也许是calligra精确指定了对应的依赖库的版本，而不是大于等于）。这下好了，我还没有办法安装了，除非我重新启用update源……
如果强制安装旧版本，会导致unity-2d,virtualbox, vlc 什么的都被卸载
$ sudo apt-get install libqtcore4=4:4.8.1-0ubuntu4.5
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
tea-data libboost-regex1.46.1 checkbox texlive-generic-extra texlive-science
antiword
libboost-signals1.46.1 lyx-common python-dateutil texlive-science-doc elyxer
Use 'apt-get autoremove' to remove them.
The following packages will be REMOVED:
appmenu-qt checkbox-qt goldendict keepassx libdbusmenu-qt2 libdconf-qt0
libphonon4 libqt4-dbus
libqt4-declarative libqt4-network libqt4-opengl libqt4-script libqt4-sql
libqt4-sql-sqlite libqt4-svg
libqt4-xml libqt4-xmlpatterns libqtbamf1 libqtdee2 libqtgconf1 libqtgui4
libqtwebkit4
libunity-2d-private0 lyx phonon phonon-backend-gstreamer qdbus qt-at-spi
sni-qt tea ubuntu-desktop
unity-2d unity-2d-panel unity-2d-shell unity-2d-spread virtualbox-4.3 vlc
The following packages will be DOWNGRADED:
libqtcore4
0 upgraded, 0 newly installed, 1 downgraded, 37 to remove and 0 not upgraded.
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