Generally, in the depot the pallets are on the ground grouped
together with others that are identical.
There is a set F = {f1, . . . , fnf} of available forklifts with the corresponding
driver that can work simultaneously. There are four types
of forklifts:
Type 1: forklifts that can manage pallets only at the first level
and at the first depth.
Type 2: forklifts that can manage pallets up to the third level
but only at the first depth.
Type 3: forklifts that can work at any level but only at the first
depth.
Type 4: forklifts that can work at any level and any depth.
Forklift types are hierarchical in the sense that the locations
reachable by a forklift of a type include the locations reachable
by a forklift of any inferior type. All forklifts work at the same
speed.