code: public void label(String labelText, int index)
  	{
  		if (labelMap.containsKey(labelText)) throw new IllegalArgumentException(
  			"Label \"" + labelText + "\" already exists.");
  
  		if (labelMap.containsValue(index)) throw new IllegalArgumentException(
  			"Index \"" + index + "\" already has a label.");
  
  		labelMap.put(labelText, index);
  	}
docs: Puts the given label for the given index.
  @param labelText the label
  @param index index to label