define a.SET.1:1 a.SET.2:2 b.SET.1:1 b.SET.2:2 c.SET.1:1 c.SET.2:2 ---- iter seek-ge a seek-ge b seek-ge c seek-ge d seek-lt a seek-lt b seek-lt c seek-lt d seek-prefix-ge a seek-prefix-ge b seek-prefix-ge c seek-prefix-ge d ---- a:2 b:2 c:2 . . a:1 b:1 c:1 a:2 b:2 c:2 . iter first next next prev prev prev next next next next next next prev prev next prev prev next next next next prev ---- a:2 a:1 b:2 a:1 a:2 . a:2 a:1 b:2 b:1 c:2 c:1 c:2 b:1 c:2 b:1 b:2 b:1 c:2 c:1 . c:1