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ObjectTracer
Last updated at 12:48 am UTC on 14 February 2003
From the class comment:

An ObjectTracer can be wrapped around another object, and then give you a chance to inspect it whenever it receives messages from the outside. For instance...

  (ObjectTracer on: Display) flash: (50@50 extent: 50@50)

will give control to a debugger just before the message flash is sent.

Obviously this facility can be embellished in many useful ways.