In all circumstances, nodes relations for tree structures are enforced. Nodes are said to cover their descendants. We may traverse a tree by going from one node to another, following node relations. A path is any sequence of nodes within the same tree. Node traversal implies a path through the tree.
All paths known for any node constitute the context of the node.
The parent, children and left and sight sibling constitute the immediate context of the node. As nodes can only access their immediate context special object should keep track of path traversal into some context rather than some immediate context. Such objects are called streams . The nodes make up the document's content . Therefore, UNIT is a subtype of NODE .