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