maintainer's note: the key observation here is that the compared
element is the first slot of the second ceil(half) of the array, and
thus can be removed for further comparison when it does not match, so
that we descend into the second ceil(half)-1 rather than ceil(half)
elements. this change ensures that nel strictly decreases with each
iteration, so that the case of != but nel==1 does not need to be
special-cased anymore.
