Lemma 4. Given a picker is at bin m in a zone and ri > rj > 0 for
m < i < j and ha > hb for some b 2 Bi, a 2 Bj, then an exchange between
the locations of items a and b will increase Pmi and decrease Pmj.
On the other hand, if b 2 Bi, a2 Bj for m > i > j, then an
exchange between the locations of items a and b, will increase Pmi
and decrease Pmj.