A B +------+ +------+ U-1 | ? | | ? | +------+ +------+ . . . . . . . . . . . . . . . . . . . . +------+ +------+ 4 | 0 | | 3 | +------+ +------+ 3 | 4 | | 1 | <---- C (== 3) +------+ +------+ 2 | 1 | | 1 | +------+ +------+ 1 | 2 | | 2 | +------+ +------+ 0 | 2 | | 4 | +------+ +------+