. . . "A person is a notified person's agent, nominee, or other person acting under the direction or control of a notified person for purposes of 26 U.S.C. 7609(e) if the person with respect to whose liability the summons is issued has the ability in fact or at law to cause the agent, etc., to take the actions permitted under 26 U.S.C. 7609(b)." .