docs: private final int load_exponent(StringBuilder sb) throws IOException
      {
          int c = read_char();
          if (c == '-' || c == '+') {
              sb.append((char)c);
              c = read_char();
          }
          c = load_digits(sb, c);
  
          if (c == '.') {
              sb.append((char)c);
              c = read_char();
              c = load_digits(sb, c);
          }
          return c;
      }