code: public Tune removeTune(int referenceNumber) {
  		Tune ret = getTune(referenceNumber);
  		if (ret != null) {
  			ret.getTuneInfos().setBookInfos(null);
  			ret.getInstructions().removeAll(getInstructions());
  			m_tunes.remove(new Integer(referenceNumber));
  		}
  		return ret;
  	}
docs: Remove the tune having the requested reference number from the book
  
  @return the removed tune, null if didn't exist.