code: public ExtendedOutputStreamWriter append(double d) throws IOException {
          super.append(String.format(Locale.ROOT, doubleFormat, d));
          return this;
      }
docs: Writes the specified double to the stream, formatted according to the format specified in the constructor.
  
  @param d the double to write to the stream
  @return this writer
  @throws IOException if an I/O error occurs