code: public static UnsignedLong valueOf(String string, int radix) {
      return fromLongBits(UnsignedLongs.parseUnsignedLong(string, radix));
    }
docs: Returns an {@code UnsignedLong} holding the value of the specified {@code String}, parsed as
  an unsigned {@code long} value in the specified radix.
  
  @throws NumberFormatException if the string does not contain a parsable unsigned {@code long}
  value, or {@code radix} is not between {@link Character#MIN_RADIX} and
  {@link Character#MAX_RADIX}