public function listSubscribe($listId, $email, Array $mergeVars = NULL, $emailType = 'text', $doubleOptin = true, $updateExisting = false, $replaceInterests = true, $sendWelcome = true) {
    $response = $this->dispatch(__FUNCTION__, array(
      'id'=>$listId,
      'email_address'=>$email,
      'email_type'=>$emailType
    ));
    
    // boolean
    if ($response->getRaw() === 'true') {
      return TRUE;
    } else {
      throw Exception::unknown($response->getRaw());
    }
  }