Base revision on a svn: external from a pre-commit hook script

[luis montes]

Is it possible to know the base revision on a svn: external from a
pre-commit hook script? If the external changes with the current
transaction I figured I could possibly parse the svnlook diff output, but
what if it does not change with the current transaction? I understand that
information is kept on the client, so does the client send it to the
server? For each external?
Thanks,
Luis