SET WINDOW -1,1,-1,1 DRAW a(18) 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)) DRAW a(n-1) WITH ROTATE(PI/4)*SCALE(1/SQR(2))*SCALE(1,-1)*SHIFT(1/2,1/2) END IF END PICTURE