public static void Reduce(uint[] xx, uint[] z)
{
ulong cc = Nat256.Mul33Add(PInv33, xx, 8, xx, 0, z, 0);
uint c = Nat256.Mul33DWordAdd(PInv33, cc, z, 0);
Debug.Assert(c == 0 || c == 1);
if (c != 0 || (z[7] == P7 && Nat256.Gte(z, P)))
{
Nat.Add33To(8, PInv33, z);
}
}