How to add, access, unset arrays in CI session?

[eluser]metaltapimenye[/eluser]
cowboy mode..

Code:

#...
        #array
        $x=array('a'=>'11212','b'=>'cccbcbcb');
        #session array assigning
        $this->session->set_userdata('mejik',$x);
        print_r($this->session->userdata('mejik'));
#result:Array ( [a] => 11212 [b] => cccbcbcb )

        $session=$this->session->userdata('mejik');
        #unset
        unset($session['a']);
        #re-set session
        $this->session->set_userdata('mejik',$session);
        print_r($this->session->userdata('mejik'));
#result:Array ( [b] => cccbcbcb )
#...