{HEAD ~ A ridge} functioning as {TAIL ~ a weir} may be formed on the back plate portion of the groove portion 
{ HEAD ~ A ridge } functioning as { TAIL ~ a weir } may be formed on the back plate portion of the groove portion
HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH