func mul51(a uint64, b uint32) (lo uint64, hi uint64) {
	mh, ml := bits.Mul64(a, uint64(b))
	lo = ml & maskLow51Bits
	hi = (mh << 13) | (ml >> 51)
	return
}

// Pow22523 set v = x^((p-5)/8), and returns v. (p-5)/8 is 2^252-3.
func (v *Element) Pow22523(x *Element) *Element {
	var t0, t1, t2 Element

	t0.Square(x)             // x^2
	t1.Square(&t0)           // x^4
	t1.Square(&t1)           // x^8