The only action taken by {TAIL ~ a prospective user} is to pull {HEAD ~ a single stair tread} down 
The only action taken by { TAIL ~ a prospective user } is to pull { HEAD ~ a single stair tread } down
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH