/**
     * Constructs a new automaton whose language is the concatenation of the language of this automaton
     * and the language of the given automaton.  [Martin, Th. 3.25]
     * The input automata are unmodified.
     * @exception IllegalArgumentException if the alphabets of <tt>f</tt> and this automaton are not the same
     */
    public NFA concat(NFA f) throws IllegalArgumentException {
        if (!alphabet.equals(f.alphabet))
            throw new IllegalArgumentException("alphabets are different");
        NFA f1 = (NFA) this.clone();
        NFA f2 = (NFA) f.clone();
        NFA n = new NFA();
        n.alphabet = alphabet;
        n.states.addAll(f1.states);
        n.states.addAll(f2.states);
        n.accept.addAll(f2.accept);
        n.initial = f1.initial;
        n.transitions.putAll(f1.transitions);
        n.transitions.putAll(f2.transitions);
        for (State s : f1.accept)
            n.addLambda(s, f2.initial);
        return n;
    }