static void LoadXml(
            string xmlFilename,
            int index,
            out int knownTabCount,
            out int unknownTabCount,
            out int ignoredTabCount
            )
        {
            // **** tell the user what's going on ****

            Console.WriteLine($"#{index, -4} - {xmlFilename}");

            // **** ****

            if (!ExcelXmlReader.ParseFile(xmlFilename, out DataSet ds, true, true))
            {
                Console.WriteLine($"Failed to parse the file: {xmlFilename}.");

                // **** nothing else to do ****

                knownTabCount   = 0;
                unknownTabCount = 0;
                ignoredTabCount = 0;

                return;
            }

            // **** create our spreadsheet object ****

            FhirSpreadsheet sheet = new FhirSpreadsheet(xmlFilename, ds);

            knownTabCount   = sheet.KnownTabs.Count;
            unknownTabCount = sheet.UnknownTabs.Count;
            ignoredTabCount = sheet.IgnoredTabs.Count;
        }