k) for the purposes of paragraph (b), w
here the value of a particular interest in a trust depends on the death of a beneficiary before attaining such age, not exceeding 40 years, as is specified in the trust instrument, and the particular
interest of the beneficiary is an
interest in possession, the value of that particular
interest and the value of any other
interest determined with reference to the value of that particular
interest shall be determined as if that beneficiary will attain the age specified in the t
...[+++]rust instrument; and