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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 BuildApprovalUrn(self, approval_id):
 """Builds approval object urn."""

    event = rdf_events.AuditEvent(
        user=self.token.username,
        action="CLIENT_APPROVAL_REQUEST",
        client=self.subject_urn,
        description=self.reason)
    events.Events.PublishEvent("Audit", event, token=self.token)

    return self.ApprovalUrnBuilder(self.subject_urn.Path(), self.token.username,
                                   approval_id)