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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_source(self, source):
 """Add a vehicle data source to the instance. The Vehicle instance will be set as the callback of the source, and the source will be started if it is startable. (i.e. it has a ``start()`` method). """

        if source is not None:
            self.sources.add(source)
            source.callback = self._receive
            if hasattr(source, 'start'):
                source.start()