src/Pure/Tools/build_job.scala

   private val session_background = deps.background(session_name)
 
   private val future_result: Future[Process_Result] =
     Future.thread("build", uninterruptible = true) {
       val parent = info.parent.getOrElse("")
-      val base = deps(parent)
       val result_base = deps(session_name)
 
       val env =
         Isabelle_System.settings(
           List("ISABELLE_ML_DEBUGGER" -> options.bool("ML_debugger").toString))