/**
     * Computes the <img src="http://cs.au.dk/~amoeller/RegAut/Lambda.gif" alt="Lambda">-closure
     * of the given state set. [Martin, Def. 3.13]
     * @param set set of {@link State} objects
     * @return set of {@link State} objects (the input set is unmodified)
     */
    public Set<State> lambdaClosure(Set<State> set) {
        Set<State> res = new HashSet<State>(); // contains the states that are in the closure and have been visited
        Set<State> pending = new HashSet<State>(); // contains the states that are in the closure but have not yet been visited
        pending.addAll(set);
        while (!pending.isEmpty()) {
            State q = pending.iterator().next();
            pending.remove(q);
            res.add(q);
            Set<State> s = new HashSet<State>(delta(q, LAMBDA));
            s.removeAll(res);
            pending.addAll(s);
        }
        return res;
    }

    /**
     * Performs transitions in extended transition function. [Martin, Def. 3.14]
     * @return <img src="http://cs.au.dk/~amoeller/RegAut/delta.gif" alt="delta">*(q,s) 
     * @exception IllegalArgumentException if a symbol in <tt>s</tt> is not in the alphabet
     */
    public Set<State> deltaStar(State q, String s) throws IllegalArgumentException {
        Set<State> set = new HashSet<State>();
        set.add(q);
        set = lambdaClosure(set);
        for (char c : s.toCharArray()) {
            Set<State> nset = new HashSet<State>();
            for (State p : set)
                nset.addAll(delta(p, c));
            set = lambdaClosure(nset);
        }
        return set;
    }