diff -r 78aa7846e91f -r d8a0e996614b src/Pure/Thy/file_format.scala
--- a/src/Pure/Thy/file_format.scala	Wed Mar 03 22:48:46 2021 +0100
+++ b/src/Pure/Thy/file_format.scala	Thu Mar 04 15:41:46 2021 +0100
@@ -39,7 +39,7 @@
       agents.mkString("File_Format.Session(", ", ", ")")
 
     def prover_options(options: Options): Options =
-      (options /: agents)({ case (opts, agent) => agent.prover_options(opts) })
+      agents.foldLeft(options) { case (opts, agent) => agent.prover_options(opts) }
 
     def stop_session: Unit = agents.foreach(_.stop)
   }