code: public void updateLabel(String oldLabel, String newLabel)
  	{
  		if (hasLabel(newLabel)) throw new IllegalArgumentException(
  			"The label \"" + newLabel + "\" already exists.");
  
  		int i = indexOf(oldLabel);
  		labelMap.remove(oldLabel);
  		labelMap.put(newLabel, i);
  	}
docs: Changes a label. The oldLabel has to be an existing label and new label has to be a new
  label.
  @param oldLabel label to update
  @param newLabel updated label