public override ECFieldElement SquarePlusProduct(ECFieldElement x, ECFieldElement y)
        {
            ulong[] ax = this.x;
            ulong[] xx = ((SecT113FieldElement)x).x, yx = ((SecT113FieldElement)y).x;

            ulong[] tt = Nat128.CreateExt64();
            SecT113Field.SquareAddToExt(ax, tt);
            SecT113Field.MultiplyAddToExt(xx, yx, tt);

            ulong[] z = Nat128.Create64();
            SecT113Field.Reduce(tt, z);
            return(new SecT113FieldElement(z));
        }