class Abstract
{
        private var sub:IAbstract;
        // Private constructor disallows instantiation
        private function Abstract()
        {
                // Cast a reference of the extending class instance to the interface type
                // which contains the abstract methods.
                sub = IAbstract(this);
                //Flash Player 7 Only
                if(sub == null) // invalid cast will return null in Flash player 7
                {
                        throw("Classes extending Abstract must implement IAbstract");
                }
                // end Flash Player 7 Only
                /* For Flash Player 6 (or 7) use this runtime type check
                if(!(sub instanceof IAbstract))
                {
                sub = null;
                trace("Classes extending Abstract must implement IAbstract");
        }
                */
                //Because sub is of type IAbstract no compiler error is thrown
                sub.myAbstractMethod();
        }
}