string toFile()
{
stringstream ss;
ss << alphabet.toString() << endl
<< nstates() << endl
<< n_accepting << endl
<< accepting() << endl
<< transition_table.toFAString();
return ss.str();
}
string toString()
{
stringstream ss;
ss << "alphabet: " << alphabet.toString() << endl
<< "nstates : " << nstates() << endl
<< "astates : " << n_accepting
<< " (" << accepting() << ')' << endl
<< "Transitions: " << endl
<< transition_table.toString() << endl;
return ss.str();
}