+define gf_inv(a, b)
+{
+ local g, x, y, X, Y, u, v, t, q, r;
+ x = gf(1); X = gf(0);
+ y = gf(0); Y = gf(1);
+
+ if (b == gf(0)) { g = a; } else if (a == gf(0)) { g = b; }
+ else {
+ while (b != gf(0)) {
+ q = gf_div(b, a); r = gf_mod(b, a);
+ t = X * q + x; x = X; X = t;
+ t = Y * q + y; y = Y; Y = t;
+ b = a; a = r;
+ }
+ g = a;
+ }
+ if (g != gf(1)) quit "not coprime in gf_inv";
+ return Y;
+}
+