((a)) ** General rule** The value of the gross estate shall include the value of all property to the extent of any interest therein of which the decedent has at any time after , made a transfer (except in case of a bona fide sale for an adequate and full consideration in money or money’s worth), by trust or otherwise, if—

((1)) possession or enjoyment of the property can, through ownership of such interest, be obtained only by surviving the decedent, and

((2)) the decedent has retained a reversionary interest in the property (but in the case of a transfer made before , only if such reversionary interest arose by the express terms of the instrument of transfer), and the value of such reversionary interest immediately before the death of the decedent exceeds 5 percent of the value of such property.

((b)) ** Special rules** For purposes of this section, the term “reversionary interest” includes a possibility that property transferred by the decedent—

((1)) may return to him or his estate, or

((2)) may be subject to a power of disposition by him,