let utf8_index_of_char s c =
let cpt = ref 0 in
let current = ref 0 in
let len = String.length s in
while !current < c && !cpt < len do
cpt := !cpt + utf8_nb_bytes_of_char s.[!cpt];
incr current;
done;
if !current = c then
!cpt
else
raise Not_found