# File lib/money/bank/variable_exchange.rb, line 215
      def export_rates(format, file = nil, opts = {})
        raise Money::Bank::UnknownRateFormat unless
          RATE_FORMATS.include? format

        store.transaction do
          s = FORMAT_SERIALIZERS[format].dump(rates)

          unless file.nil?
            File.open(file, "w") {|f| f.write(s) }
          end

          s
        end
      end