At the root of the tree should be a
Gtk2::Window, which can contain a single child.
Normally the window will contain either a VBox or
HBox. In the example above a VBox is used. That
VBox has two children the first of which is a
frame and the last of which is a Label. The label
has no children and therefore is a leaf node in
the widget hierarchy. A Frame is a simple widget
with a text label and outline box, it can contain
a single widget. The frame in this case has the
text 'Button' and contains a HBox. In that HBox
are two buttons one with the text 'Click Me' and
the other with the text 'Quit'. Both of these
buttons are leaf nodes in that they contain no
other widgets. A simplistic tree graph of this
widget hierarchy is shown above.


$vbox can contain multiple widgets

A Frame is a simple widget with a text label 
and outline box, it can contain a single widget.