Môi trường tham khảo tĩnh:
	+ main: a//1 , b//1 , c//1, main, sub1//2
	+ sub1: a//2, b//1, c//1, sub1//2, sub2//3, sub3//4, main
	+ sub2: a//3, c//3, f//3, b//1, sub2//3, sub3//4, main, sub1//2
	+ sub3: a//2, c//1, b//4, sub2//3, sub3//4, main, sub1//2