{HEAD ~ The opening} has {TAIL ~ a bottom boundary} whose height is adjustable, so that it can be adjusted to lie above the surface of the glass melt 
has
{ HEAD ~ The opening } has { TAIL ~ a bottom boundary } whose height is adjustable , so that it can be adjusted to lie above the surface of the glass melt
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL 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