src/Pure/Tools/simplifier_trace.scala

--- a/src/Pure/Tools/simplifier_trace.scala	Wed Aug 26 15:59:21 2020 +0100
+++ b/src/Pure/Tools/simplifier_trace.scala	Thu Aug 27 12:34:10 2020 +0200
@@ -326,6 +326,6 @@
           false
       }
 
-    val functions = List(Markup.SIMP_TRACE_CANCEL -> cancel)
+    override val functions = List(Markup.SIMP_TRACE_CANCEL -> cancel)
   }
 }