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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:
 async def _get_rev_reg_def(self, rr_id: str) -> str: """ Get revocation registry definition from ledger by its identifier. Raise AbsentRevReg for no such revocation registry, logging any error condition and raising BadLedgerTxn on bad request. Retrieve the revocation registry definition from the agent's revocation cache if it has it; cache it en passant if it does not (and such a revocation registry definition exists on the ledger). :param rr_id: (revocation registry) identifier string, of the format '<issuer-did>:4:<issuer-did>:3:CL:<schema-seq-no>:<tag>:CL_ACCUM:<tag>' :return: revocation registry definition json as retrieved from ledger """


        LOGGER.debug('_BaseAgent._get_rev_reg_def >>> rr_id: %s', rr_id)

        rv_json = json.dumps({})

        with REVO_CACHE.lock:
            revo_cache_entry = REVO_CACHE.get(rr_id, None)
            rr_def = revo_cache_entry.rev_reg_def if revo_cache_entry else None
            if rr_def:
                LOGGER.info('_BaseAgent._get_rev_reg_def: rev reg def for %s from cache', rr_id)
                rv_json = json.dumps(rr_def)
            else:
                get_rrd_req_json = await ledger.build_get_revoc_reg_def_request(self.did, rr_id)
                resp_json = await self._submit(get_rrd_req_json)
                try:
                    (_, rv_json) = await ledger.parse_get_revoc_reg_def_response(resp_json)
                    rr_def = json.loads(rv_json)
                except IndyError:  # ledger replied, but there is no such rev reg
                    LOGGER.debug('_BaseAgent._get_rev_reg_def: <!< no rev reg exists on %s', rr_id)
                    raise AbsentRevReg('No rev reg exists on {}'.format(rr_id))

                if revo_cache_entry is None:
                    REVO_CACHE[rr_id] = RevoCacheEntry(rr_def, None)
                else:
                    REVO_CACHE[rr_id].rev_reg_def = rr_def

        LOGGER.debug('_BaseAgent._get_rev_reg_def <<< %s', rv_json)
        return rv_json