are shown below, where classes doing work behind the scenes are colored in grey.

\image html tree_building_classes.jpg "Tree Building Classes"

The different classes has different responsibilities:
- Qtilities::Core::Observer : A normal Observer class which allows attachment of items to tree nodes. It is also responsible to monitor the modification state of TreeNode as well as the exportability.
- Qtilities::CoreGui::AbstractTreeItem : A base class which is implemented by all tree items and nodes. This cla