Compiler Error. Incompatible Override overriding double extended method

1

Why this won’t work?

MyClass_1


public class MyClass_1 extends MovieClip{
    public function MyClass_1 () {

    }

    public function show():void {

    }
}

MyClass_2


public class MyClass_2 extends MyClass_1{
    public function MyClass_2 () {

    }

    override public function show():void {
        gotoAndPlay("in");
    }
}

MyClass_3


public class MyClass_3 extends MyClass_2 {
    public function MyClass_3 () {

    }

    override public function show():void {
        trace("here will be incompatible override");
    }
}

when tring to compile i get Incompatible override in MyClass_3.
All classes are in different packages…
WHY??