diff -r 5e312d6bb883 -r 6bf6160a2c54 src/Pure/Tools/build.scala
--- a/src/Pure/Tools/build.scala	Thu Mar 04 16:08:30 2021 +0100
+++ b/src/Pure/Tools/build.scala	Thu Mar 04 16:23:34 2021 +0100
@@ -8,7 +8,7 @@
 package isabelle
 
 
-import scala.collection.SortedSet
+import scala.collection.immutable.SortedSet
 import scala.annotation.tailrec