My point is just that, given a situation where we need to merge two revs where 
one is a descendant of the other, can we not safely assume that simply 
choosing the descendant is the right thing to do? After all, given revs a and 
b with chosen common ancestor x, we are looking for a version y that 
incorporates both the changes of a vs x and of b vs x. But in this case, if a 
is a descendant of b, then a already contains the changes of b vs x, so 
choosing it seems the proper thing to do.