can't install kde with apt-get

[Matteo Croce]

KDE can't be installed with apt-get since the 'kde' metapackage depends on old kde3-only packages (kdeaddons)

[~]# apt-get install kde
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.

Since you only requested a single operation it is extremely likely that
the package is simply not installable and a bug report against
that package should be filed.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
  kde: Depends: kdeaddons (>= 4:3.5.5) but it is not installable
E: Broken packages
[~]# apt-get install kdeaddons
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package kdeaddons is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package kdeaddons has no installation candidate
[~]# apt-cache show kdeaddons
[~]# apt-cache show kde |grep ^Depends
Depends: kde-core (>= 5:47), kdeaccessibility (>= 4:3.5.5), kdeaddons (>= 4:3.5.5), kdeadmin (>= 4:3.5.5), kdeartwork (>= 4:3.5.5), kdeedu (>= 4:3.5.5), kdegames (>= 4:3.5.5), kdegraphics (>= 4:3.5.5), kdemultimedia (>= 4:3.5.5), kdenetwork (>= 4:3.5.5), kdepim (>= 4:3.5.5), kdetoys (>= 4:3.5.5), kdeutils (>= 4:3.5.5), kdewebdev (>= 4:3.5.5)