Use authz to allow user access to only a specific subdirectory

[Andrew Martin <am...@xes-inc.com>]

Hello,

I am running Subversion 1.7.7 on Ubuntu 12.04 using apache2 + dav_svn +
mod-auth-ntlm-winbind for authentication. In my apache config, I specify an
authz file for access control:
AuthzSVNAccessFile      /path/to/authz

In the repository, I have several different sub-directories:
/trunk/a
/trunk/b
/trunk/c

I would like to grant user1 read access to only /trunk/a, but not access to
/trunk/b or /trunk/c (or other future sibling directories that are created). The
examples in the SVN Book show how to grant access if the user already has access
to the parent directory, but not how to grant access to only a specific
sub-directory. Using the following authz syntax doesn't work (user1 is denied 
access to /trunk/a):
[Repo:/]
* = 
user2 = r

[Repo:/trunk/a]
user1 = r

How can I grant access only to a specific sub-directory?

Thanks,

Andrew Martin