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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 get_submission_ids(self, tournament=1):
 """Get dict with username->submission_id mapping. Args: tournament (int):
 ID of the tournament (optional, defaults to 1) Returns: dict: username->submission_id mapping, string->string Example: {'1337ai': '93c46857-fed9-4594-981e-82db2b358daf', '1x0r': '108c7601-822c-4910-835d-241da93e2e24', } """

        query = """
            query($tournament: Int!) {
              rounds(tournament: $tournament
                     number: 0) {
                leaderboard {
                  username
                  submissionId
                }
            }
        }
        """
        arguments = {'tournament': tournament}
        data = self.raw_query(query, arguments)['data']['rounds'][0]
        if data is None:
            return None
        mapping = {item['username']: item['submissionId']
                   for item in data['leaderboard']}
        return mapping