For purposes of giving notice, waiving notice, initiating a proceeding, granting consent or approval, or objecting with regard to any proceedings under this chapter, the sole holder or all co-holders of a presently exercisable or testamentary general power of appointment, power of revocation, or unlimited power of withdrawal are deemed to represent and act for beneficiaries to the extent that their interests as permissible appointees, takers in default, or otherwise are subject to the power.