The present invention relates to an item of luggage incorporating a collapsed resting support which when erected may be used to support portions of a resting person in a forwards leaning seated position. The output and inverting input of amplifiers are connected by a respective capacitor. The method employs an additional bit to represent the modules result and may also utilize a load store number mapping memory to increase execution speed. Due to this construction a leakage gas flow out of the first gas chamber of the first main valve leads, if the first servo valve fails, to an increase in pressure in the first gas chamber of the second main valve, whereby it is ensured that the main valve is safely closed. Refrigerant pressure is sensed at a predetermined location in the refrigeration system.