[r-t] 1278 Major

[Joe Norton]

Is there any reason why there couldn't be a Differential Major method 
which was just 1278? i.e., the plain course would be:
12345678
12436578
12345678

It seems to be on a similar theme to Cross Differential Major 
(http://bb.ringingworld.co.uk/view.php?id=919487). But I can imagine 
that there may be rules that say something along the lines of "all bells 
must change place at least once during the plain course". Is there such 
a rule?

Thanks,
Joe.