code: @Nullable
    public static BigInteger parseBigInteger (@Nullable final String sStr)
    {
      return parseBigInteger (sStr, DEFAULT_RADIX, null);
    }
docs: Parse the given {@link String} as {@link BigInteger} with radix
  {@value #DEFAULT_RADIX}.
  
  @param sStr
  The String to parse. May be <code>null</code>.
  @return <code>null</code> if the string does not represent a valid value.