(taxpayer proposed to disclaim contingent right to receive trust corpus upon trust termination; disclaimer to be executed within nine months after reaching age 18; disclaimer determined to be made within timeframe of Treas. Reg. Sec. 25.2511-1(c); IRS noted that disclaimer must be unequivocal and be effective under local law; IRS determined that, based on facts represented, disclaimer would not be a taxable transfer).