{HEAD ~ The material} of which {TAIL ~ the covers} 30 are formed should have the same characteristics mentioned above in connection with the embodiment of FIGS 1 and 2 and should be slightly stressed when secured in place 
{ HEAD ~ The material } of which { TAIL ~ the covers } 30 are formed should have the same characteristics mentioned above in connection with the embodiment of FIGS 1 and 2 and should be slightly stressed when secured in place
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