Basic OOP question

In the Library, I Have an mc named ‘block’ which is being attached to the Stage. Inside this MC, is another MC called ‘inside_mc’. ‘block’ has a alphaClass.as associated with it.
My class code looks as follows

class ButtonClass extends MovieClip {
    public function ButtonClass() {
        this.onRollOver = this.over;
    }
    private function over() {
        this.inside_mc._alpha = 50;
    }
}

When the movie publishes, flash says that the MC has no property with the name ‘inside_mc’, thus does not work.
Can someone point out where I am going wrong please? Because, really… The MC does have the property ‘inside_mc’!
Thanks …/:cowbell: