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