A controller, consisting in {HEAD ~ part} of a single panel or {TAIL ~ a series} of panels, is provided to selectively open and close the apertures in response to the orientation of the eraser as it contacts the chalkboard 
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A controller , consisting in { HEAD ~ part } of a single panel or { TAIL ~ a series } of panels , is provided to selectively open and close the apertures in response to the orientation of the eraser as it contacts the chalkboard
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL 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