The Secretary may prescribe regulations providing that where, by reason of varying rates of interest, put or call options, indefinite maturities, contingent payments, assumptions of debt instruments, or other circumstances, the tax treatment under this subpart (or section
163
(e)) does not carry out the purposes of this subpart (or section
163
(e)), such treatment shall be modified to the extent appropriate to carry out the purposes of this subpart (or section
163
(e)).