+ * The #GtkTreeModel interface contains two methods for reference
+ * counting: gtk_tree_model_ref_node() and gtk_tree_model_unref_node().
+ * These two methods are optional to implement. The reference counting
+ * is meant as a way for views to let models know when nodes are being
+ * displayed. #GtkTreeView will take a reference on a node when it is
+ * visible, which means the node is either in the toplevel or expanded.
+ * Being displayed does not mean that the node is currently directly
+ * visible to the user in the viewport. Based on this reference counting
+ * scheme a caching model, for example, can decide whether or not to cache
+ * a node based on the reference count. A file-system based model would
+ * not want to keep the entire file hierarchy in memory, but just the
+ * folders that are currently expanded in every current view.
+ *
+ * When working with reference counting, the following rules must be taken
+ * into account:
+ * <itemizedlist>
+ * <listitem><para>Never take a reference on a node without owning a
+ * reference on its parent. This means that all parent nodes of a referenced
+ * node must be referenced as well.</para></listitem>
+ * <listitem><para>Outstanding references on a deleted node are not released.
+ * This is not possible because the node has already been deleted by the
+ * time the row-deleted signal is received.
+ * </para></listitem>
+ * <listitem><para>Models are not obligated to emit a signal on rows of
+ * which none of its siblings are referenced. To phrase this differently,
+ * signals are only required for levels in which nodes are referenced. For
+ * the root level however, signals must be emitted at all times (however the
+ * root level is always referenced when any view is attached).
+ * </para></listitem>
+ * </itemizedlist>