src/Pure/PIDE/xml.scala

--- a/src/Pure/PIDE/xml.scala	Mon Mar 20 14:36:15 2017 +0100
+++ b/src/Pure/PIDE/xml.scala	Mon Mar 20 15:37:14 2017 +0100
@@ -240,6 +240,8 @@
 
     /* representation of standard types */
 
+    val tree: T[XML.Tree] = (t => List(t))
+
     val properties: T[Properties.T] =
       (props => List(XML.Elem(Markup(":", props), Nil)))
 
@@ -322,6 +324,12 @@
 
     /* representation of standard types */
 
+    val tree: T[XML.Tree] =
+    {
+      case List(t) => t
+      case ts => throw new XML_Body(ts)
+    }
+
     val properties: T[Properties.T] =
     {
       case List(XML.Elem(Markup(":", props), Nil)) => props