src/Pure/Build/build_schedule.scala

         Library.symmetric_difference(read_scheduled_builds_domain(db), running_builds_domain)
 
       remove_schedules(db, remove)
     }
 
-    override val tables = SQL.Tables(Schedules.table, Nodes.table)
+    override val tables: SQL.Tables = SQL.Tables(Schedules.table, Nodes.table)
 
     val all_tables: SQL.Tables =
       SQL.Tables.list(Build_Process.private_data.tables.list ::: tables.list)
   }