protected void addPreviousTrailer(PTrailer previousTrailer) {
//System.out.println("PTrailer.addPreviousTrailer()");
        getPrimaryCrossReference().addToEndOfChainOfPreviousXRefs(previousTrailer.getPrimaryCrossReference());

        // Later key,value pairs take precedence over previous entries
        Hashtable currDictionary = getDictionary();
        Hashtable prevDictionary = previousTrailer.getDictionary();
        Enumeration prevKeys = prevDictionary.keys();
        while (prevKeys.hasMoreElements()) {
            Object prevKey = prevKeys.nextElement();
            if (!currDictionary.containsKey(prevKey)) {
                Object prevValue = prevDictionary.get(prevKey);
                currDictionary.put(prevKey, prevValue);
            }
        }
    }