/* return This mod modulus, N is modulus, ND is Montgomery Constant */
public FF reduce(FF N, FF ND)
{ // fast karatsuba Montgomery reduction
int n = N.length;
FF t = new FF(2 * n);
FF r = new FF(n);
FF m = new FF(n);
r.sducopy(this);
m.karmul_lower(0, this, 0, ND, 0, t, 0, n);
karmul_upper(N, m, t, n);
m.sducopy(this);
r.add(N);
r.sub(m);
r.norm();
return(r);
}