consider mathML comments as text content

stevencohn#1150

OneMore/Commands/Clean/RemoveEmptyCommand.cs

@@ -9,6 +9,7 @@ namespace River.OneMoreAddIn.Commands
 	using System.Collections.Generic;
 	using System.Globalization;
 	using System.Linq;
+	using System.Text.RegularExpressions;
 	using System.Threading.Tasks;
 	using System.Windows.Forms;
 	using System.Xml.Linq;
@@ -122,7 +123,7 @@ public bool CollapseEmptyLines()
 			var elements = range
 				.Select(e => e.Parent)
 				.Distinct()
-				.Where(e => e.TextValue().Trim().Length == 0)
+				.Where(e => e.TextValue().Trim().Length == 0 && !e.IsMathML())
 				.ToList();
 
 			if (elements?.Any() != true)

OneMore/Helpers/Extensions/XElementExtensions.cs

@@ -427,6 +427,20 @@ public static XElement FirstAncestor(this XElement element, XName name, XName br
 		}
 
 
+		/// <summary>
+		/// Determines if the content of the element consists only of a mathML statement
+		/// which is identified when its CDATA specifies an XML comment.
+		/// </summary>
+		/// <param name="element"></param>
+		/// <returns></returns>
+		public static bool IsMathML(this XElement element)
+		{
+			var cdata = element.GetCData();
+			return cdata != null &&
+				Regex.IsMatch(cdata.Value, @"<!--.+?-->", RegexOptions.Singleline);
+		}
+
+
 		/// <summary>
 		/// OneMore Extension >> Extract the sanitized text value of the given element
 		/// </summary>