diff -r 5e312d6bb883 -r 6bf6160a2c54 src/Pure/Thy/sessions.scala
--- a/src/Pure/Thy/sessions.scala	Thu Mar 04 16:08:30 2021 +0100
+++ b/src/Pure/Thy/sessions.scala	Thu Mar 04 16:23:34 2021 +0100
@@ -11,7 +11,7 @@
 import java.nio.channels.FileChannel
 import java.nio.file.StandardOpenOption
 
-import scala.collection.{SortedSet, SortedMap}
+import scala.collection.immutable.{SortedSet, SortedMap}
 import scala.collection.mutable