Systems and methods for reducing interference between multiple infra-red depth cameras are described. The fan retention shaft has a first shaft which in use extends from the engine and a second shaft to which in use the fan is attached, the second shaft being coaxial with the first shaft and the first and second shafts defining an annular passage between them. The fragment program may include one or more opcodes. Upon failure of an index node, the failure may be detected at other nodes, including those nodes that store the replicas of the object.