code: public byte[] sendGenericNack(OutputStream os, int commandStatus,
              int sequenceNumber) throws IOException {
          byte[] b = pduComposer.genericNack(commandStatus, sequenceNumber);
          writeAndFlush(os, b);
          return b;
      }
docs: /*
  (non-Javadoc)
  
  @see org.jsmpp.PDUSender#sendGenericNack(java.io.OutputStream, int, int)