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