SIP and its inverse

One of the many unfortunate things about the TAN-SIP polynomial-distorted WCS convention is that it carries around parameters for forward and reverse directions, even though these can be derived from one another; indeed, an iterated reverse operation can be correct to machine precision, whereas the polynomial description of the inverse of a polynomial distortion can never be correct to machine precision. But anyway, since we are going to use TAN-SIP (dammit), I wrote code today to make the reverse coefficients with as much precision as possible. It works, but it is not yet ready for prime time.