{HEAD ~ a pair} of pinions, {TAIL ~ each} having an outer teeth gear thereon; and 
{ HEAD ~ a pair } of pinions , { TAIL ~ each } having an outer teeth gear thereon ; and
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH