public GrammarRule this[int from, int to] {
     get { Cardinality = new Cardinality(from, to); return this; }
 }
 public GrammarRule(GrammarElement element) { Name = "Implicit"; Element = element; Cardinality = 1; }
 public GrammarRule(string name, GrammarElement element) { Name = name; Element = element; Cardinality = 1; }
 public GrammarRule(string name) { Name = name; Cardinality = 1; }