| http://www.w3.org/ns/prov#value | - In another aspect of the invention, one of the walls, preferably upper wall 2, is made of semi-rigid plastic material that has a weakened region 5, such as by scores or perforations as discussed above, and the other of the walls, such as lower wall 2???, is made of a film or thin sheet, preferably flexible, that generally does not withstand tearing and thus does not require its own specific weaken
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