public <PSC> void removeVanishObserver(PSC ptn) {
   Observer.remove(_names.get(ptn));
   _names.remove(ptn);
   _vanishOb.remove(ptn);
   _state.remove(ptn);
 }
 public void removeChangeObserver(int threshold) {
   Observer.remove(_cnames.get(threshold));
   _names.remove(threshold);
   _changeOb.remove(new Integer(threshold));
   _minChanges = getMinChanges();
 }
 public <PSC> void removeAppearObserver(PSC ptn) {
   Observer.remove(_names.get(ptn));
   _names.remove(ptn);
   _appearOb.remove(ptn);
   _state.remove(ptn);
 }