src/Pure/General/file_watcher.scala

 class File_Watcher private[File_Watcher]  // dummy template
 {
   def register(dir: JFile) { }
   def register_parent(file: JFile) { }
   def deregister(dir: JFile) { }
+  def purge(retain: Set[JFile]) { }
   def shutdown() { }
 }
 
 object File_Watcher
 {

           case Some(key) =>
             key.cancel
             st.copy(dirs = st.dirs - dir)
         })
 
+    override def purge(retain: Set[JFile]): Unit =
+      state.change(st =>
+        st.copy(dirs = st.dirs --
+          (for ((dir, key) <- st.dirs.iterator if !retain(dir)) yield { key.cancel; dir })))
+
 
     /* changed directory entries */
 
     private val delay_changed = Standard_Thread.delay_last(delay)
     {