Explanation: Debian wheezy
Package: *
Pin: release o=Debian,n=wheezy
Pin-Priority: 990

Explanation: Debian wheezy-updates
Package: *
Pin: release o=Debian,n=wheezy-updates
Pin-Priority: 990

Explanation: Debian sid
Package: *
Pin: release o=Debian,n=sid
Pin-Priority: 1

Explanation: Debian squeeze
Package: *
Pin: release o=Debian,n=squeeze
Pin-Priority: 980

Explanation: Debian fallback
Package: *
Pin: release o=Debian
Pin-Priority: -10