public Grammar(Grammar parent, ExpBuilder builder) : base(null)
 {
     this.Parent  = parent;
     this.Builder = builder;
 }
 public Grammar(ExpBuilder builder) : this(null, builder)
 {
 }
 public Grammar(string name, Expression top,
                ExpBuilder builder, Grammar parent) : base(name, top)
 {
     this.Parent  = parent;
     this.Builder = builder;
 }