let starts_with ~what ?(offset=0) str =
let what_idx = ref 0 in
let str_idx = ref offset in
let ok = ref true in
while !ok &&
!str_idx < String.length str &&
!what_idx < String.length what do
if str.[!str_idx] = what.[!what_idx] then
incr what_idx
else
ok := false;
incr str_idx
done;
if !what_idx = String.length what then
true
else
false