How to use the pouchdb-errors.REV_CONFLICT function in pouchdb-errors

txn.get(stores.localStore, doc._id, function (err, resp) {
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return callback(err);
        } else {
          return callback(pouchdbErrors.createError(pouchdbErrors.MISSING_DOC));
        }
      }
      if (resp._rev !== doc._rev) {
        return callback(pouchdbErrors.createError(pouchdbErrors.REV_CONFLICT));
      }
      txn.batch([{
        prefix: stores.localStore,
        type: 'del',
        key: doc._id
      }]);
      var ret = {ok: true, id: doc._id, rev: '0-0'};
      if (opts.ctx) {
        // don't execute immediately
        return callback(null, ret);
      }
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }

txn.get(stores.localStore, doc._id, function (err, resp) {
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return callback(err);
        } else {
          return callback(pouchdbErrors.createError(pouchdbErrors.MISSING_DOC));
        }
      }
      if (resp._rev !== doc._rev) {
        return callback(pouchdbErrors.createError(pouchdbErrors.REV_CONFLICT));
      }
      txn.batch([{
        prefix: stores.localStore,
        type: 'del',
        key: doc._id
      }]);
      var ret = {ok: true, id: doc._id, rev: '0-0'};
      if (opts.ctx) {
        // don't execute immediately
        return callback(null, ret);
      }
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }