SET bitmap SIZE 500,250
FOR d=0 TO 8
   SET WINDOW 0,3^d,0,3^d/2
   DRAW a(d)
NEXT d
END
EXTERNAL PICTURE a(n)
IF n=0 THEN
   PLOT LINES:0,0;1,0
END IF
IF n>0 THEN
   LET k=3^(n-1)
   DRAW a(n-1)
   DRAW a(n-1) WITH ROTATE(PI/2)*SHIFT(k,0)
   DRAW a(n-1) WITH SHIFT(k,k)
   DRAW a(n-1) WITH ROTATE(3*PI/2)*SHIFT(2*k,k)
   DRAW a(n-1) WITH SHIFT(2*k,0)
END IF
END PICTURE