Hi nusenu,
Oh, I didn't know about this feature since it is not documented in the man page, but great to see that it exists. It fits my use-case very well!
*sigh* this is undocumented indeed. Bug #12698 (https://bugzilla.ipfire.org/show_bug.cgi?id=12698) has been raised for fixing this.
Thanks, and best regards, Peter Müller