@Override
    public boolean add(T value) {
        boolean result = super.add(value);
		if ( result ) {  // only track if new element not in set
			elements.add(value);
		}
		return result;
    }
Add a value to list; keep in hashtable for consistency also;
 Key is object itself.  Good for say asking if a certain string is in
 a list of strings.
runtime/Java/src/org/antlr/v4/runtime/misc/OrderedHashSet.java