(decedent's will gave residue of estate to trust for surviving spouse; trust split into two trusts with one trust to pay surviving spouse income for life then to spouse's estate upon death with trust terminating and remaining balance passing to Family Trust that would pay to daughter for life and then to daughter's issue; estate tax return timely filed and estate elected to treat trust property as QTIP, but did not indicate trust was to be severed into exempt and non-exempt trust; no reverse QTIP election made; IRS granted 120 days to sever trust into exempt and non-exempt trust and make reverse QTIP election).