private List<Statement> helpIntersect(List<Statement> a, List<Statement> b) {
        List<Statement> result = new LinkedList<Statement>();
        if(a == null || b == null)
            return result;
        else if(a.isEmpty() || b.isEmpty())
            return result;
        else {
            for(Statement e: a)
                result.add(e);
            for(Statement e: a)
                if(!b.contains(e))
                    result.remove(e);
        }
        return result;
    }