Doc is pretty sketchy. A, B = bilinear (a, b, fs) a, b are numerator, denominator, respectively? each are polynomials in _descending_ negative powers of s? e.g.: a = a0 + a1 * s**-1 + a2 * s**-2 ... A, B are numerator, denominator, respectively? each are polynomials in descending negative powers of z? e.g.: A = A0 + A1 * z**-1 + A2 * z**-2 ...