$ sudo apt-get install gnat-4.6
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.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 base-files : PreDepends: awk
 gnat-4.6 : Depends: gcc-4.6 (>= 4.6.0-12) but it is not going to be installed
            Depends: libgnat-4.6 (= 4.6.1-4ubuntu3) but it is not going to be installed
            Depends: libc6-dev (>= 2.13-0ubuntu6) but it is not going to be installed
            Depends: libc6 (>= 2.2.4) but it is not going to be installed
            Depends: libgcc1 (>= 1:4.1.1) but it is not going to be installed
            Depends: libgmp10 but it is not going to be installed
            Depends: libgnatprj4.6 (>= 4.6.1-4ubuntu3) but it is not going to be installed
            Depends: libgnatvsn4.6 (>= 4.6.1-4ubuntu3) but it is not going to be installed
            Depends: libmpc2 but it is not going to be installed
            Depends: libmpfr4 but it is not going to be installed
            Depends: zlib1g (>= 1:1.1.4) but it is not going to be installed
            PreDepends: multiarch-support but it is not going to be installed
 libsdl1.2debian : Depends: libsdl1.2debian-alsa (= 1.2.14-6.1ubuntu4) but it is not going to be installed or
                            libsdl1.2debian-all (= 1.2.14-6.1ubuntu4) but it is not going to be installed or
                            libsdl1.2debian-esd (= 1.2.14-6.1ubuntu4) but it is not going to be installed or
                            libsdl1.2debian-oss (= 1.2.14-6.1ubuntu4) but it is not going to be installed or
                            libsdl1.2debian-nas (= 1.2.14-6.1ubuntu4) but it is not going to be installed or
                            libsdl1.2debian-pulseaudio (= 1.2.14-6.1ubuntu4) but it is not going to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.