several rev, one patch exclude branch

[Uwe Brauer]

Hello

Please have a lock at the following log -G

@  changeset:   6831:51db987e1c4b
|  bookmark:    master
|  tag:         tip
|  parent:      6828:812891d581e1
|  user:        Uwe Brauer <oub at mat.ucm.es>
|  date:        Sat Aug 13 19:25:53 2016 +0000
|  summary:     * exam.el (LaTeX-exam-class-options): Typos
|
| o  changeset:   6830:d933bd3edf00
| |  bookmark:    uwe
| |  user:        Uwe Brauer <oub at mat.ucm.es>
| |  date:        Sat Aug 13 16:19:35 2016 +0000
| |  summary:     Start to rewrite section about prettifying in doc/auctex.texi
| |
| o  changeset:   6829:2eeafa7e08a6
|/   user:        Uwe Brauer <oub at mat.ucm.es>
|    date:        Sat Aug 13 16:17:16 2016 +0000
|    summary:     Add style/bidi.el
|
o  changeset:   6828:812891d581e1
|  user:        Uwe Brauer <oub at mat.ucm.es>
|  date:        Sat Aug 13 15:55:06 2016 +0000
|  summary:     Add entry to Makefile.in: examn.el (found in style/examn.el)
|

I want to generate a patch which covers rev 6828 till 6831 but excludes
6830 and 6829 because it is on a branch.
How can I do that?
hg export -g -r 6831:6828

Generated a patch which included the branch, which is not what I want

Regards

Uwe Brauer