Own regression from rB019ce363b01bba0af, not totally sure why removing the heading
zero value in element_sum broke binary search distribution in random branch, but...
New code makes thing sooo much simpler, by simply getting rid of all zero-weight
items in element_sum (at the cost of adding a temporary extra index mapping array),
that way neither random nor 'even ordered' distribution code has to wory anymore
about skipping zero-weighted items!
abf6f9f6cf