code: public void setPrefix(String key, String prefix) {
  		PdfName fieldname = new PdfName(key);
  		PdfObject o = get(fieldname);
  		if (o == null)
  			throw new IllegalArgumentException("You must set a value before adding a prefix.");
  		PdfDictionary dict = new PdfDictionary(PdfName.COLLECTIONSUBITEM);
  		dict.put(PdfName.D, o);
  		dict.put(PdfName.P, new PdfString(prefix, PdfObject.TEXT_UNICODE));
  		put(fieldname, dict);
  	}
docs: Adds a prefix for the Collection item.
  You can only use this method after you have set the value of the item.
  @param prefix	a prefix