An anaerobic digester is fed a feedstock, for example sludge from a municipal wastewater treatment plant, and produces a digestate. A blade holder 13 including a plurality of blades 16 vertically installed is held in such a manner as to be freely advanceable and retractable in an up-and-down direction. Then the CPU is tested separately. The approach is applicable to all database systems and other servers which support storing and managing XML content.