src/Pure/PIDE/markup_tree.scala

--- a/src/Pure/PIDE/markup_tree.scala	Thu Aug 19 22:04:20 2010 +0200
+++ b/src/Pure/PIDE/markup_tree.scala	Thu Aug 19 22:26:15 2010 +0200
@@ -50,6 +50,12 @@
 {
   import Markup_Tree._
 
+  override def toString =
+    branches.toList.map(_._2) match {
+      case Nil => "Empty"
+      case list => list.mkString("Tree(", ",", ")")
+    }
+
   def + (new_node: Node): Markup_Tree =
   {
     branches.get(new_node) match {