Privatizing Form Validation callback functions?

[eluser]inktri[/eluser]
For example:

Code:

username_check($str)

from http://www.codeignitor.com/user_guide/li...ation.html can be accessed by "/form/username_check" right?

How would you prevent that? I tried adding an underscore in front of username_check and calling callback__username_check with the validation rules with no luck