private void writeNetscapeExtension(OutputStream os) throws IOException
      {
        // n.b. most software seems to interpret the count as a repeat count
        // (i.e., interations beyond 1) rather than as an iteration count
        // (thus, to avoid repeating we have to omit the whole extension)
        
        os.write((int) '!');           // GIF Extension Introducer
        os.write(0xff);             // Application Extension Label
        
        os.write(11);                 // application ID block size
        putAscii("NETSCAPE2.0", os);  // application ID data
        
        os.write(3);                   // data sub-block size
        os.write(1);                   // a looping flag? dunno
    
        // we finally write the relevent data
        putShort(loopCount > 1 ? loopCount - 1 : 0, os); 
        
        os.write(0);                   // block terminator
      }