The stair tread mechanism is connected with a window lock and {HEAD ~ initial movement} of the stairs unlocks {TAIL ~ the window} 
unlocks
The stair tread mechanism is connected with a window lock and { HEAD ~ initial movement } of the stairs unlocks { TAIL ~ the window }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL