let find ?(from=0) ?length s ~f =
let length_of_s = S.length s in
let from, length = resize_from_length ~from ?length ~length_of_s in
let found = ref None in
let i = ref 0 in
while !found = None && !i < length do
if f (get_exn s (!i + from))
then found := Some (!i + from)
else incr i
done;
!found