This section aims to give the problem statement with the related assumptions.
In particular, the allocation of units of different product types,
distinguished according to their physical features and grouped into classes
for a better management in the warehouse, is taken into account. Although
the compatibility among the products of the same class is always satisfied,
the one among items of different groups could not be verified. For example,
foodstuffs and detersives can be allocated neither in the same slots nor in
the adjacent ones. In addition, the warehouse also presents a multi-layer
layout that assures a three-dimensional aspect to the problem. Each slot,
in fact, has a width, a length and a height. Thus, a block indicates a set of
overlapped slots.