}

    @Override
    public boolean contains(final Object o) {
        return map.containsKey(o);
    }

    @Override
    public boolean add(final String o) {
        return map.put(o, PRESENT) == null;
    }

    @Override
    public boolean remove(final Object o) {
        return map.remove(o) == PRESENT;
    }

    @Override
    public void clear() {
        map.clear();
    }