typestate in std::str::replace

gistfile1.txt

fn replace(s: &istr, from: &istr, to: &istr) : is_not_empty(from) -> istr {
    if byte_len(s) == 0u {
        ret ~"";
    } else if starts_with(s, from) {
        check (is_not_empty(from));
        ret to + replace(slice(s, byte_len(from), byte_len(s)), from, to);
    } else {
        check (is_not_empty(from));
        ret unsafe_from_byte(s[0]) +
                replace(slice(s, 1u, byte_len(s)), from, to);
    }
}