X1 X1 X2 X22 2 X X X X X X  1 2 1 X 1 2   2 2 2 2X X 1 X X 1  1 1 2 2

2)

x

1

x

1

x

2

x

2

2

2

x

x

x x

x

x

1

2

1

x

1

2

2

2

2

2

x

x

1

x

x

1

1

1

2

2