src/Pure/PIDE/markup_tree.scala

                   if (body.isDefinedAt(entry._1)) rest else rest + entry)
               else branches
             new Markup_Tree(rest, Entry(new_markup, new Markup_Tree(body)))
           }
           else { // FIXME split markup!?
-            System.err.println("Ignored overlapping markup information: " + new_markup)
+            System.err.println("Ignored overlapping markup information: " + new_markup +
+              body.filter(e => !new_range.contains(e._1)).mkString("\n"))
             this
           }
         }
     }
   }