The device includes a continuous track and {HEAD ~ a ball} which is movable within and around {TAIL ~ the track} 
The device includes a continuous track and { HEAD ~ a ball } which is movable within and around { TAIL ~ the track }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL