{HEAD ~ A controller}, consisting in {TAIL ~ part} of a single panel or 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 
{ HEAD ~ A controller } , consisting in { TAIL ~ part } of a single panel or 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
HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL 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 OTH OTH OTH OTH OTH OTH OTH