Why does nor work my validation rule?

[eluser]oliwin[/eluser]
Hello!
At page i have input box:
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<input 13px;" name="rules" id="User_agreement" value="1" type="checkbox">

<?=form_error('rules')?>


And at side php there is validation code:

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<?$this->form_validation->set_rules('rules', 'Согласие с правилами', 'required|callback__rules_accept');

function _rules_accept($check){

if($this->input->post('rules') && $this->input->post('rules') == 1) {
return true;
}

$this->form_validation->set_message('_email_unic', 'Необходимо подтвердить ваше согласие с правилами использования');
return false;
}
?>

But it does not work! Why? Can you explain me?