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In Smalltalk Express, the ListBox class happens to be a direct subclass of
the ControlPane class, which, in turn, is a direct subclass of
the SubPane class, which is a superclass of all widgets. The
ControlPane class is a superclass of all widgets that are
implemented by the operating system; other SubPane classes
implement widgets in Smalltalk itself.
*-------------* | SubPane | *-------------* | | | | extends | *-------------*---> v extends v *-------------* *-------------* | ControlPane | | GraphPane | *-------------* *-------------* | | | | extends | *-------------*---> v extends v *-------------* *-------------* | ListBox | | Button | *-------------* *-------------*