src/Pure/Tools/build_job.scala

   def join: Process_Result = Process_Result.undefined
   def make_abstract: Build_Job.Abstract = Build_Job.Abstract(job_name, numa_node)
 }
 
 object Build_Job {
-  case class Abstract(
+  sealed case class Abstract(
     override val job_name: String,
     override val numa_node: Option[Int]
   ) extends Build_Job {
     override def make_abstract: Abstract = this
   }