public void toggleAction(Action a, boolean is) {
    if (a == null) return ;
    if (! a.actor.inWorld()) {
      I.complain(a+" TOGGLED BY DEFUNCT ACTOR: "+a.actor) ;
    }
    
    final Target t = a.target() ;
    List <Action> forT = actions.get(t) ;
    if (is) {
      if (forT == null) actions.put(t, forT = new List <Action> ()) ;
      forT.include(a) ;
    }
    else if (forT != null) {
      forT.remove(a) ;
      if (forT.size() == 0) actions.remove(t) ;
    }
  }