It is clear that the time to pick a batch of n orders changes
according to their storage locations in the warehouse. However,
in this model, we assume that the picking tour time of n items,
T(n), is an increasing function of the number of items, n, and independent
of their locations. In other words, we assume that T(n)
represents the average picking time of n items. Still, we assume
that T(n) is a concave function of n and therefore there is a motivation
for batching items before going on a tour.