public void TestAccepts09() {
var productions = new HashSet<Production> {
CFGParser.Production("<S> -> ε"),
// CFGParser.Production("<S> -> 'x'"),
CFGParser.Production("<S> -> <S> <S> <S>"),
};
Grammar g = new Grammar(productions, Nonterminal.Of("S"));
CNFGrammar h = g.ToCNF();
Assert.IsTrue(h.Accepts(Sentence.FromLetters("")));
}