A long expression that indicates the node's level.
The Level property indicates the node's level. A root node
has the level 0. The child nodes of the root node has the level 1, and so on.
The level of the child nodes is equal with the level of the parent node plus
1. Use the LevelWidth property to specify the level's width.
Use the VisibleLevelCount property
to specify the number of levels being displayed.
