Each stem is movable between a withdrawn position and an extended position with respect to the base, such that the second end projects from the base when {HEAD ~ the stem} is in {TAIL ~ the extended position} 
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Each stem is movable between a withdrawn position and an extended position with respect to the base , such that the second end projects from the base when { HEAD ~ the stem } is in { TAIL ~ the extended position }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL