{HEAD ~ at least one pair} of {TAIL ~ wheel driving means} adapted to be driven independently of each other 
of
{ HEAD ~ at least one pair } of { TAIL ~ wheel driving means } adapted to be driven independently of each other
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH