protected function checkParams($params,$operator)
  {
    if (!is_array($params)) {
    	throw new Exception('Web2All_Table_MultiValueOperator: first param must be an array');
    }
    if ($operator=='IN') {
    	if (count($params)<1) {
    		throw new Exception('Web2All_Table_MultiValueOperator: IN operator requires at least one param');
    	}
    } elseif ($operator=='BETWEEN') {
    	if (count($params)!=2) {
    		throw new Exception('Web2All_Table_MultiValueOperator: BETWEEN operator requires exactly two params');
    	}
    }else{
      throw new Exception('Web2All_Table_MultiValueOperator: unsupported operator "'.$operator.'"');
    }
  }