public override void Visit(DelimiterNode node)
 {
   switch (node.Type) {
   case SqlDelimiterType.Column:
     AppendLine(node.Text);
     AppendIndent();
     break;
   default:
     Append(node.Text);
     break;
   }
 }
 public virtual void Visit(DelimiterNode node)
 {
     throw new InvalidOperationException();
 }