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