code: func Sum(m []byte, key nacl.Key) *[Size]byte {
  	mac := hmac.New(sha512.New, (*key)[:])
  	mac.Write(m)
  	sum := mac.Sum(nil) // first 256 bits of 512 bit sum
  	out := new([32]byte)
  	copy(out[:], sum[:32])
  	return out
  }
docs: // Sum generates an authenticator for m using a secret key and returns the
  // 32-byte digest.