/**
     * Performs transitions in extended transition function. [Martin, Def. 2.12]
     * @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 State deltaStar(State q, String s) throws IllegalArgumentException {
        // (Using recursion instead of iteration would have been closer to 
        // the mathematical definition, but this code is simpler...)
        for (char c : s.toCharArray()) 
            q = delta(q,c);
        return q;
    }