private static ITry <TaxSummary, IEnumerable <string> > GetV4TaxSummary(Dto.Archive archive) { var invoiceFooter = archive.InvoiceFooter.ToTry(_ => "Invoice footer file not found.".ToEnumerable()); return(invoiceFooter.FlatMap(f => { var taxBreakdownNet = Try.Aggregate(f.Rows.Select(row => ParseLineTaxSummary(row.Values[1]))); var taxBreakdownTax = Try.Aggregate(f.Rows.Select(row => ParseLineTaxSummary(row.Values[2]))); return Try.Aggregate( taxBreakdownNet, taxBreakdownTax, (net, tax) => TaxSummary.Sum(net.Concat(tax)) ); })); }