public static MvcHtmlString ToHtmlString( Next expression, bool link = true)
 {
     return MvcHtmlString.Create (string.Format("○ {0}",
                                                Embed(expression, expression.Enclosed, expression.Enclosed.ToHtmlString(link))
                                                ));
 }
 private static string ToHtmlString(Next expression)
 {
     return string.Format("○ {0}",
                          Parenthesize(expression, expression.Enclosed, ToHtmlString(expression.Enclosed))
                          );
 }