In {HEAD ~ the rotary version}, {TAIL ~ the vent follower} operates in a wiper type fashion 
In { HEAD ~ the rotary version } , { TAIL ~ the vent follower } operates in a wiper type fashion
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH