public void TestAccepts06() {
var productions = new HashSet<Production> {
CFGParser.Production("<X_3> -> ε"),
CFGParser.Production("<X_2> -> ε"),
CFGParser.Production("<X_0> -> <X_2> <X_1>"),
CFGParser.Production("<X_1> -> <X_3> <X_3>"),
};
var g = new Grammar(productions, Nonterminal.Of("X_0"));
var h = g.ToCNF();
Assert.IsTrue(h.Accepts(Sentence.FromWords("")));
}