/**
     * Looks up transitions 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 Set<State> delta(State q, char c) throws IllegalArgumentException {
        if (!alphabet.symbols.contains(c) && c!=LAMBDA)
            throw new IllegalArgumentException("symbol '"+c+"' not in alphabet");
        Set<State> set = transitions.get(new StateSymbolPair(q, c));
        if (set==null)
            set = new HashSet<State>();
        return set;
    }