The base is perforated by at least one port whose length is very much less than the mean diameter and the base, external to the box is capped by a flexible and sealed bellows adapted to form with it {HEAD ~ a pocket} of {TAIL ~ variable volume} 
The base is perforated by at least one port whose length is very much less than the mean diameter and the base , external to the box is capped by a flexible and sealed bellows adapted to form with it { HEAD ~ a pocket } of { TAIL ~ variable volume }
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