anchor.diff:  }
   
   // A series of terms matched one after the other.
  -class Alternative extends BinaryAst{
  +class Alternative extends BinaryAst {
     Alternative(Ast l, Ast r) : super(l, r);
   
     String toString() => "($l$r)";
  
positive.code:     print("}");
    }
    String toString() => "[${ranges.join()}]";
  }
  
  class Start extends Ast {
  
negative.code:     print("}");
    }
    bool isAnchored() => true;
    int get minWidth => 0;
    int get maxWidth => 1;
  }
  
  // Single char superclass.
  