The power of a trustee under the Bankruptcy Code to reverse certain transactions or liens which occurred before the filing of the bankruptcy case (§ 544(a)(1), Bankruptcy Code). The trustee is considered an ideal hypothetical lien creditor with a judgment who may contest the validity of unperfected liens. The trustee is also considered a hypothetical, bona fide purchaser of real property of the bankruptcy estate and may avoid obligations of the debtor that are voidable by such a purchaser.