DocumentTraversal contains methods that create
TreeWalkers to traverse a
node and its children in document order (depth first, pre-order
traversal, which is equivalent to the order in which the start tags occur
in the text representation of the document).
||Filters are objects that know how to "filter out" nodes.
NodeIterators are used to step through a set of nodes, e.g.
TreeWalker objects are used to navigate a document tree or
subtree using the view of the document defined by their
whatToShow flags and filter (if any).