code: function getNextAndPreviousTransactions(numTransactionsInLedger, callback) {
      async.concat([false, true], function(earliestFirst, concat_callback) {
        var params = {
          account: notificationDetails.account,
          max: numTransactionsInLedger + 1,
          min: numTransactionsInLedger + 1,
          limit: numTransactionsInLedger + 1,
          earliestFirst: earliestFirst
        };
  
        // In rippled -1 corresponds to the first or last ledger
        // in its database, depending on whether it is the min or max value
        if (params.earliestFirst) {
          params.ledger_index_max = -1;
          params.ledger_index_min = notificationDetails.transaction.ledger_index;
        } else {
          params.ledger_index_max = notificationDetails.transaction.ledger_index;
          params.ledger_index_min = -1;
        }
  
        transactions.getAccountTransactions(api, params, concat_callback);
  
      }, callback);
  
    }
docs: Query for one more than the numTransactionsInLedger going forward and backwards to get a range of transactions that will definitely include the next and previous transactions