string_ref next() const {
     const int pos = src_.substr(pos_).find_first_of(sep_);
     const int old_pos = pos_;
     if (pos != -1)
         pos_ += pos + 1;
     else
         finished_ = true;
     return src_.substr(old_pos, pos);
 }
 string_ref rest() const { return src_.substr(pos_); }