(decedent's retained control over portion of property given to son via implied agreement resulted in only the value of the portion of the property decedent retained possession or enjoyment over being included in decedent's estate; Tax Court opinion requiring full value of transferred property to be included in estate vacated; IRS argument in case contrary to IRS's own position taken in Treas. Reg. Sec. 20.2036-1(c)(1)(i) and Rev. Rul. 79-109; terms of any implied agreement through which decedent retained control matter; case remanded to Tax Court).