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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 changes(self):
 """ Returns a dict containing just the fields which have changed on this Document since it was created or last saved, together with both their previous and current values doc['name'] # => 'bob' doc['name'] = 'clive' doc.changes # => {'name': ('bob', clive')} """

        return {key: (self._previous[key], self._instance[key])
                for key in self._previous}