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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 create_volume(self, availability_zone, size=None, snapshot_id=None):
 """Create a new volume."""

        params = {"AvailabilityZone": availability_zone}
        if ((snapshot_id is None and size is None) or
            (snapshot_id is not None and size is not None)):
            raise ValueError("Please provide either size or snapshot_id")
        if size is not None:
            params["Size"] = str(size)
        if snapshot_id is not None:
            params["SnapshotId"] = snapshot_id
        query = self.query_factory(
            action="CreateVolume", creds=self.creds, endpoint=self.endpoint,
            other_params=params)
        d = query.submit()
        return d.addCallback(self.parser.create_volume)