Lemma 3. Given a picker is at bin m in a zone and ra > rb > 0, for some
a 2 Bi, b 2 Bj for m < i < j, then an exchange between the items in bin i
and bin j will increase Pmj and decrease Pmi. On the other hand, if
a 2 Bi, b 2 Bj for m > i > j, then an interchange between the items in
bin i and bin j will increase Pmj and decrease Pmi.