src/Pure/PIDE/markup_tree.scala

       make_elems(elem_markup, body.toList)
     }
    make_body(root_range, Nil, overlapping(root_range))
   }
 
-  override def toString =
+  override def toString: String =
     branches.toList.map(_._2) match {
       case Nil => "Empty"
       case list => list.mkString("Tree(", ",", ")")
     }
 }