/**
     * Looks up transition in transition function.
     * @return <img src="http://cs.au.dk/~amoeller/RegAut/delta.gif" alt="delta">(q,c) 
     * @exception IllegalArgumentException if <tt>c</tt> is not in the alphabet
     */
    public State delta(State q, char c) throws IllegalArgumentException {
        if (!alphabet.symbols.contains(c))
            throw new IllegalArgumentException("symbol '"+c+"' not in alphabet");
        return transitions.get(new StateSymbolPair(q, c));
    }