(a) In general In the case of any debt instrument to which this section applies, for purposes of this subpart, the issue price shall be- (1) where there is adequate stated interest, the stated principal amount, or (2) in any other case, the imputed principal amount. (b) Imputed principal amount For purposes of this section- (1) In general Except as provided in paragraph (3), the imputed principal amount of any debt instrument shall be equal to the sum of the present values of all payments due under