{HEAD ~ a means} to rotate said {TAIL ~ control ring} so as to change the position of said vent 
to rotate
{ HEAD ~ a means } to rotate said { TAIL ~ control ring } so as to change the position of said vent
HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH