What effect does rdiff-backup's --exclude option have on previous increments?

Using rdiff-backup, a reverse-incremental backup tool, if I use the --exclude option to exclude some files that were previously included in earlier increments (older backups), what effect will it have?

It looks like the excluded file disappears from the latest version.

$ ls -l original/
total 0
-rw-r--r-- 1 jkiv jkiv 0 2010-11-14 16:35 a.txt
-rw-r--r-- 1 jkiv jkiv 0 2010-11-14 16:35 b.txt
-rw-r--r-- 1 jkiv jkiv 0 2010-11-14 16:35 c.txt
$ ls -l backup/
total 0
$ rdiff-backup original/ backup/
$ ls -l backup/
total 4
-rw-r--r-- 1 jkiv jkiv    0 2010-11-14 16:35 a.txt
-rw-r--r-- 1 jkiv jkiv    0 2010-11-14 16:35 b.txt
-rw-r--r-- 1 jkiv jkiv    0 2010-11-14 16:35 c.txt
drwx------ 3 jkiv jkiv 4096 2010-11-14 18:16 rdiff-backup-data
$ echo "Update for good measure." > original/a.txt 
$ rdiff-backup --exclude original/b.txt original/ backup/
$ ls -l backup/
total 8
-rw-r--r-- 1 jkiv jkiv   42 2010-11-14 18:16 a.txt
-rw-r--r-- 1 jkiv jkiv    0 2010-11-14 16:35 c.txt
drwx------ 3 jkiv jkiv 4096 2010-11-14 18:17 rdiff-backup-data