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