U

a 1

a 2

a 3

a 4

d

x 1

1

1

3

2

1

x 2

2

*

2

1

1

x 3

1

1

3

2

1

x 4

1

*

*

1

2

x 5

*

2

2

*

2

x 6

1

2

*

2

2

x 7

2

2

2

1

2

x 8

1

2

1

1

1