src/Pure/General/exn.scala

--- a/src/Pure/General/exn.scala	Sat Aug 25 17:20:06 2018 +0200
+++ b/src/Pure/General/exn.scala	Sat Aug 25 20:10:49 2018 +0200
@@ -138,4 +138,10 @@
 
   def message(exn: Throwable): String =
     user_message(exn) getOrElse (if (is_interrupt(exn)) "Interrupt" else exn.toString)
+
+
+  /* trace */
+
+  def trace(exn: Throwable): String =
+    exn.getStackTrace.mkString("\n")
 }