def add_companion(self, bill_id, session=None, chamber=None):
companion = {'<STR_LIT>': bill_id,<EOL>'<STR_LIT>': session or self['<STR_LIT>'],<EOL>'<STR_LIT>': chamber}<EOL>self['<STR_LIT>'].append(companion)<EOL>
Associate another bill with this one.

If session isn't set it will be set to self['session'].