/**
  * @return Itk_Pgm_Result
  */
 public function execute()
 {
     if (!$this->_adapter instanceof Itk_Connection_AdapterAbstract) {
         throw new Itk_Connection_Exception('Program object must be called from $adapter->getProgram("PGMNAM")');
     }
     $result = $this->_adapter->execute($this);
     if (!$result instanceof Itk_Pgm_Result) {
         throw new Itk_Pgm_Exception('Invalid result type');
     }
     return $result;
 }