--- 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