/** 
     * Constructs a new automaton with the same language as this automaton but without unreachable states. 
     * The input automaton is unmodified.
     */
    public FA removeUnreachableStates() {
        FA f = (FA) clone();
        Set<State> reachable = f.findReachableStates();
        f.states.retainAll(reachable);
        f.accept.retainAll(reachable);
        for (StateSymbolPair sp : new HashSet<StateSymbolPair>(f.transitions.keySet())) {
            if (!f.states.contains(sp.state))
                f.transitions.remove(sp);
        }
        return f;
    }