14 protected $valid =
true;
17 function __construct(&$master = null){
18 $this->master = isset($master) ? $master : $this;
26 protected function invalidate(){
27 if ( isset($this->master) && $this->master != $this ) $this->master->invalidate();
31 function setMaster($master){
32 $this->master = $master;
35 public function getMaster(){