private List<Statement> helpUnion(List<Statement> a, List<Statement> b) {
        List<Statement> result = new LinkedList<Statement>();
        if(a != null)
            for (Statement l: a)
                result.add(l);
        if(b != null)
            for(Statement l: b)
                if(!result.contains(l))
                    result.add(l);
        return result;
    }