SET WINDOW -0.5,1.5,-1,1
DRAW a(19)
END
EXTERNAL PICTURE a(n)
IF n=0 THEN
   PLOT LINES:0,0;1,0
ELSE
   DRAW a(n-1) WITH ROTATE(-PI/4)*SCALE(1/SQR(2))*SHIFT(1/2,1/2)
   DRAW a(n-1) WITH ROTATE(-3*PI/4)*SCALE(1/SQR(2))*SHIFT(1/2,1/2)
END IF
END PICTURE