+=item csv_export_for_tax_accountant
+
+Generates up to four downloadable csv files containing data about sales and
+purchase invoices, and their respective payments:
+
+Example:
+ my $startdate = DateTime->new(year => 2012, month => 1, day => 1);
+ my $enddate = DateTime->new(year => 2012, month => 12, day => 31);
+ SL::DATEV->new(from => $startdate, to => $enddate)->csv_export_for_tax_accountant;
+ # {
+ # 'download_token' => '1488551625-815654-22430',
+ # 'filenames' => [
+ # 'Zahlungen Kreditorenbuchungen 2012-01-01 - 2012-12-31.csv',
+ # 'Kreditorenbuchungen 2012-01-01 - 2012-12-31.csv',
+ # 'Zahlungen Debitorenbuchungen 2012-01-01 - 2012-12-31.csv',
+ # 'Debitorenbuchungen 2012-01-01 - 2012-12-31.csv'
+ # ]
+ # };
+
+
+=item check_vcnumbers_are_valid_pk_numbers
+
+Returns 1 if all vcnumbers are suitable for the DATEV export, 0 if not.
+
+Finds the default length of charts (e.g. 4), adds 1 for the pk chart length
+(e.g. 5), and checks the database for any customers or vendors whose customer-
+or vendornumber doesn't consist of only numbers with exactly that length. E.g.
+for a chart length of four "10001" would be ok, but not "10001b" or "1000".
+
+All vcnumbers are checked, obsolete customers or vendors aren't exempt.
+
+There is also no check for the typical customer range 10000-69999 and the
+typical vendor range 70000-99999.
+
+=item check_valid_length_of_accounts
+
+Returns 1 if all currently booked accounts have only one common number length domain (e.g. 4 or 6).
+Will throw an error if more than one distinct size is detected.
+The error message gives a short hint with the value of the (at least)
+two mismatching number length domains.
+