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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 to_pb(self):
 """Render a protobuf message. Returns: google.iam.policy_pb2.Policy: a message to be passed to the ``set_iam_policy`` gRPC API. """


        return policy_pb2.Policy(
            etag=self.etag,
            version=self.version or 0,
            bindings=[
                policy_pb2.Binding(role=role, members=sorted(self[role]))
                for role in self
            ],
        )