+=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.