Configurable package hierarchies

  This document describes a mechanism for allowing aptitude and other APT
frontends to dynamically generate a package hierarchy using information
stored on the system.  The mechanism allows packages to be placed in arbitrary
groups and groups to be placed within arbitrary other groups.

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                             Abstract structure

  The hierarchy information is input as a directed acyclic graph.  Each node
of the graph is either a "group" or a "package"; only "group" nodes may have
successors.  The user may define one or more groups as the starting point(s)
for the displayed hierarchy; the program will then generate a tree from the
graph using those nodes as the top-level nodes in the tree.

  This is not a terribly complex system with tons of research behind it;
however, it IS very simple (trivial, in fact) to implement, and will easily
beat the pants off the horrendous Section-based grouping that we are stuck
with today.

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                                 File format

  Hierarchy information is stored in a tagfile format similar to that
used by dpkg.  (this is done primarily because apt has simple facilities for
parsing such files)  Each record in the tagfile after the first will have one
of two formats:

  (a) Package records

Package: foo
Parents: group1,group2,...,groupN

  (b) Group records

Group: bar
Description: A holding pen for foos.
Parents: group1,group2,...,groupN

  In each case, the first line identifies the package or group for which
information is being entered, while successive lines provide information
about the package or group.  It is an error if both a Group and a Package
tag exist in a given record.

  References are resolved after all data has been read (ie, references
to not-yet-defined groups will work)  Dangling references are allowed;
if an item is placed into a group which does not exist at all, that
reference will simply be ignored.  Circular references will generate
an error.

  Package names must be valid package names for the Debian package
database.  If a name which specifies a package not in the current APT
cache is encountered, it will be ignored.
  Group names must follow the same rules as package names, with the
exception that they may contain any number of periods separating
groups of alphanumerics/hyphens.  (so "dnb.lisp" is a valid group)

  Note: groups and packages have entirely disjoint namespaces.  It is
valid (albiet confusing) for a group "linux-doc" and a package
"linux-doc" to coexist.

  In the future, the Description field may have an extended component,
interpreted in the same way as the extended Package description.  It
may also become possible to override package descriptions from within
a hierarchy definition.

  One exception to this format exists.  The first record in a file may have
the format:

Global: yes
Realm: britannia

  In this case, all future group names in the file which do not
include a "." will have "britannia." prepended to their name.  This is
a mechanism to avoid namespace collisions.

  Future global file options may be added.