the extension upper surface is flush with the upper surface when {HEAD ~ the extension member} is at {TAIL ~ the first position} 
is at
the extension upper surface is flush with the upper surface when { HEAD ~ the extension member } is at { TAIL ~ the first position }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL