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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 transitions(self, issue, id=None, expand=None):
 """Get a list of the transitions available on the specified issue to the current user. :param issue: ID or key of the issue to get the transitions from :param id: if present, get only the transition matching this ID :param expand: extra information to fetch inside each transition """

        params = {}
        if id is not None:
            params['transitionId'] = id
        if expand is not None:
            params['expand'] = expand
        return self._get_json('issue/' + str(issue) + '/transitions', params=params)['transitions']