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The use of the svnmerge script to help pick off the revisions to merge.
The bottom of this link some example usage of the svnmerge.py script
http://gcc.gnu.org/wiki/SvnBranch
Useful links
http://www.asterisk.org/developers/svn-branching-merging
http://www.reactos.org/wiki/index.php/Best_practices_for_working_with_branches
http://svn.apache.org/repos/asf/httpd/httpd/trunk/VERSIONING
http://svn.collab.net/viewcvs/svn/trunk/contrib/client-side/
http://www.freebsd.org/doc/en_US.ISO8859-1/articles/releng/index.html