/* this*=y mod p */
public void modmul(FF y, FF p, FF nd)
{
//FF d=new FF(2*p.length);
long ex = P_EXCESS();
long ey = y.P_EXCESS();
if ((ex + 1) * (ey + 1) + 1 >= P_FEXCESS)
{
mod(p);
}
FF d = mul(this, y);
copy(d.reduce(p, nd));
}