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Config specs
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A *config spec* is a set of instructions for assembling a source tree
from separately maintained branches.  This is like CVS's "modules"
format.  Config specs are very commonly used in Clearcase, and the
main way of getting a new checkout.

Scenarios:

 * One module is shared by several projects, and it needs to be
   present in a subdirectory of their trees to build.

 * There are different commit or stability rules for different parts
   of a tree.

The simplest form is just a specification of branch sources, and where
they should be assembled into the tree.  Example::

  .              http://foo.net/fooapp
  ./libbaz       http://foo.net/libbaz

Normally the entire branch will be placed at this point, but sometimes
subdirectory should be selected.  Selecting just a single file rather
than subdirectory might be problematic because we wouldn't have
anywhere to put the control files.

Each branch will have a .bzr directory at the top level.  Changes on
each branch can be pushed back independently.

The tool needs to support (and be tested with) nested checkouts.
Commands should normally only apply to a single tree, but there might
be an option to descend into nested trees.

One very important use case is a naive user checking out a tree that
happens to use a configspec.  This is transparent in cvs, but not at
all so in Arch.

The ``svn:externals`` mechanism in Subversion works like this.

One interesting idea from Aaron is that all branches should be like
this: the latest-patch pointer can be generalized to in fact name
several patches that should be assembled in subdirectories.  A
simplest branch names just one, assembled to the base directory.  You
might want to specify the parent branches for each one as well....