Prevent a callback function from being public

[eluser]thisischris[/eluser]
In the user guide, under form validation there is this example:

Code:

<?php

class Form extends CI_Controller {

function index()
{
                ... ... ...
}

function username_check($str)
{
  if ($str == 'test')
  {
   $this->form_validation->set_message('username_check', 'The %s field can not be the word "test"');
   return FALSE;
  }
  else
  {
   return TRUE;
  }
}

}
?>

However how can I prevent the username_check being accessible from example.com/index.php/form/username_check

Adding a underscore to the method's name prevents it from being called when setting a rule.
Example: $this->form_validation->set_rules('username', 'username', 'callback_username_check|required');