src/Pure/PIDE/xml.scala

--- a/src/Pure/PIDE/xml.scala	Tue Aug 12 15:31:24 2014 +0200
+++ b/src/Pure/PIDE/xml.scala	Tue Aug 12 15:46:20 2014 +0200
@@ -21,7 +21,7 @@
 
   type Attributes = Properties.T
 
-  sealed abstract class Tree { override def toString = string_of_tree(this) }
+  sealed abstract class Tree { override def toString: String = string_of_tree(this) }
   case class Elem(markup: Markup, body: List[Tree]) extends Tree
   {
     def name: String = markup.name