public async Task MergeRatesAsync(ExchangeRates toMerge, CancellationToken cancellationToken)
        {
            using (await fLock.LockForWritingAsync(cancellationToken))
            {
                var newRates = new ExchangeRates(fRates);
                foreach (var rate in toMerge)
                {
                    newRates.Add(rate.Key, rate.Value);
                }

                fRates = newRates;
            }
        }