public void Empty()
{
NFA nfa = NFA.Empty();
DFA dfa = nfa.ToDfa();
Assert.IsTrue(dfa.IsMatch(""));
Assert.IsFalse(dfa.IsMatch("a"));
Assert.IsFalse(dfa.IsMatch("b"));
Assert.IsFalse(dfa.IsMatch("ab"));
Assert.IsFalse(dfa.IsMatch("aaaa"));
Assert.IsFalse(dfa.IsMatch("bbb"));
}