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From the entire list of receiver–message pairs that you see in the walkback
window, you see that an error occurs during the evaluation of the +
method defined for the Weight class, and that the error causes
doesNotUnderstand: to be sent to a SmallInteger instance.
Then, during the evaluation of the doesNotUnderstand: method defined
for Object instances, error: is sent to a
SmallInteger instance, and that causes the walkback window to appear.
All this activity occurs after the Doit message is sent to the
expression, 3gm + 2, in response to right > Show It. The
expression never produces an answer; hence, the expression is
represented in the walkback window as an instance of
UndefinedObject:
SmallInteger(Object)>>error: SmallInteger(Object)>>doesNotUnderstand: Weight>>+ UndefinedObject>>Doit