n : [xs x_s] [ys y_s] = <A> [ys y_s] [xs x_s]</>

xs : "x" [xs x_s] = <X></> [xs x_s]
   : "x"          = <X></>
   
ys : "y" [ys y_s] = <Y> [ys y_s] </>
   : "y"          = <Y></>