A matrix of {HEAD ~ apertures} in {TAIL ~ the front wall} are selectively openable and closeable by a controller in response to the orientation of the eraser 
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A matrix of { HEAD ~ apertures } in { TAIL ~ the front wall } are selectively openable and closeable by a controller in response to the orientation of the eraser
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH