Stored at e.g. /etc/apt/preferences.d/alpha_beta_rc_vers.pref.
Explanation: Prevent packages from upgrading from fully-released versions to alpha,
Explanation: beta, or release-candidate versions, unless specifically requested on the
Explanation: command line. Packages can still upgrade to these versions in certain
Explanation: situations, e.g. if a beta version is installed and an RC version is
Explanation: available, since they'll both have priority 50.
Explanation: The syntax for pattern matching is described in 'man apt_preferences',