docs: def _write_header(name, header, required, stream, encoder, strict=False):
      """
      Write AMF message header.
  
      @param name: Name of the header.
      @param header: Header value.
      @param required: Whether understanding this header is required (?).
      @param stream: L{BufferedByteStream<pyamf.util.BufferedByteStream>} that
          will receive the encoded header.
      @param encoder: An encoder capable of encoding C{AMF0}.
      @param strict: Use strict encoding policy. Default is C{False}. Will write
          the correct header length after writing the header.
      """
      stream.write_ushort(len(name))
      stream.write_utf8_string(name)
  
      stream.write_uchar(required)
      write_pos = stream.tell()
  
      stream.write_ulong(0)
      old_pos = stream.tell()
      encoder.writeElement(header)
      new_pos = stream.tell()
  
      if strict:
          stream.seek(write_pos)
          stream.write_ulong(new_pos - old_pos)
          stream.seek(new_pos)