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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_companion(self, bill_id, session=None, chamber=None):
 """ Associate another bill with this one. If session isn't set it will be set to self['session']. """

        companion = {'bill_id': bill_id,
                     'session': session or self['session'],
                     'chamber': chamber}
        self['companions'].append(companion)