private void ShowHierarchy(XmlSchemaSet schemaSet, TextWriter writer)
        {
            var schema = schemaSet.Schemas(Constants.InvoiceTargetNamespace).Cast <XmlSchema>().Single();

            prefixLookup = schema.Namespaces.ToArray().Where(v => !string.IsNullOrEmpty(v.Name)).ToDictionary(k => k.Namespace, v => v.Name);
            PrintSchemaDetail(schema, writer);
            var prefixes = schemaSet.GetNamespacePrefixes();

            Console.WriteLine();
            foreach (var prefix in prefixes)
            {
                writer.WriteLine($"{prefix.Name,9}: -> \"{prefix.Namespace}\"");
            }
        }