public void AddBubble(ChatBubble bubble)
 {
     var root = discussion.Root [0];
     if (bubble.IsLeft) {
         LeftThinking = false;
         if (RightThinking) {
             root.Insert (root.Count - 1, bubble);
         } else {
             root.Add (bubble);
         }
     } else {
         RightThinking = false;
         if (LeftThinking) {
             root.Insert (root.Count - 1, bubble);
         } else {
             root.Add (bubble);
         }
     }
 }