protected virtual Expression VisitListing(ListingExpression node)
 {
     Visit(node.Left);
     Append(",", false);
     Visit(node.Right);
     return(node);
 }
 protected virtual Expression VisitListing(ListingExpression node)
 {
     Visit(node.Left);
     Append(",", false);
     Visit(node.Right);
     return node;
 }