let uid_counter =
let counter = String.make uid_digits '0' in
let inc () =
let i = ref (uid_digits - 1) in
while (!i >= 0) && (String.get counter (!i) = 'z') do
String.set counter (!i) '0' ;
i := !i - 1
done ;
String.set counter (!i) (match String.get counter (!i) with
| '9' -> 'A'
| 'Z' -> 'a'
| c -> char_of_int (int_of_char c + 1));
String.copy counter in
inc