source 2006 Gaudry "Variants of the Montgomery form based on Theta functions", page 23/52, with A^2/B^2 = (a^2+b^2)/(a^2-b^2) as on page 20/52, replacing incorrect B^2/A^2 on page 23/52 with correct A^2/B^2 and a/b; or 2009 Gaudry--Lubicz "The arithmetic of characteristic 2 Kummer surfaces and of elliptic Kummer lines", Section 6.2, replacing incorrect A'/B' = (a^2+b^2)/(a^2-b^2) with correct A'^2/B'^2 = (a^2+b^2)/(a^2-b^2), replacing A'^2/B'^2 with A^2/B^2, and replacing z... with y...; plus notation changes: a/b and A^2/B^2 defined as 1/sqrt(r) and (1+r)/(1-r), input x^2/y^2 etc. replaced by r Z2^2/Y2^2 and r Z3^2/Y3^2 and r Z1^2/Y1^2, intermediate x'/y' replaced by W/V, output X/Y replaced by sqrt(r) Z5/Y5 parameter s assume s = (1+r)/(1-r) compute V = s(r Z2^2-Y2^2)(r Z3^2-Y3^2) compute W = (r Z2^2+Y2^2)(r Z3^2+Y3^2) compute Y5 = r Z1(W-V) compute Z5 = Y1(W+V)