{HEAD ~ The covers} enclose {TAIL ~ a cartridge extension mechanism} capable of extending the pen cartridge from the housing upon application of a pressure to the respective covers, and retracting the pen cartridge into the housing upon removal of the pressure from the covers 
{ HEAD ~ The covers } enclose { TAIL ~ a cartridge extension mechanism } capable of extending the pen cartridge from the housing upon application of a pressure to the respective covers , and retracting the pen cartridge into the housing upon removal of the pressure from the covers
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