when said support frame is upwardly opened by a predetermined angle from {HEAD ~ the closed position} via {TAIL ~ said hinge means}, said coupling shaft is pressed to the lower ends of said oblong holes while being urged by said urging means, so that said rear side frame is supported by said support frame 
when said support frame is upwardly opened by a predetermined angle from { HEAD ~ the closed position } via { TAIL ~ said hinge means } , said coupling shaft is pressed to the lower ends of said oblong holes while being urged by said urging means , so that said rear side frame is supported by said support frame
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