code: public void writeMapBegin(String type)
          throws IOException
      {
          os.write('M');
          os.write('t');
          printLenString(type);
      }
docs: Writes the map header to the stream.  Map writers will call
  <code>writeMapBegin</code> followed by the map contents and then
  call <code>writeMapEnd</code>.
  
  <code><pre>
  Mt b16 b8 (<key> <value>)z
  </pre></code>