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699: Sidetrip

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      | 
*-------------*   *-------------*