d0 := *d
	hash := d0.constSum()
	return append(in, hash[:]...)
}

func (d *digest) constSum() [Size]byte {
	var length [8]byte
	l := d.len << 3
	for i := uint(0); i < 8; i++ {
		length[i] = byte(l >> (56 - 8*i))
	}

	nx := byte(d.nx)
	t := nx - 56                 // if nx < 56 then the MSB of t is one
	mask1b := byte(int8(t) >> 7) // mask1b is 0xFF iff one block is enough