protected function expects($key, $not = null) {
		if (!array_key_exists($key, $this->strategy)) {
			trigger_error($this->name." config parameter for \"$key\" expected.", E_USER_ERROR);
			exit();
		}

		$value = $this->strategy[$key];
		if (empty($value) || $value == $not) {
			trigger_error($this->name." config parameter for \"$key\" expected.", E_USER_ERROR);
			exit();
		}

		return $value;
	}