When a new object (Btree fragment) is created, and we switch to it, we don't modify the extent lists at all.  So, if we allocate 10 blocks in object 0001, then for object 0002, we have recorded that we've used blocks 0-9, and if we allocate 10 more, we'll get 10-19.  So when we write out object 0002, there are no conflicts, but blocks 0-9 will be empty.